I work in a lab that does global optimization of mixed-integer and non-convex problems. My experience with open source optimization solvers has been that the better ones are typically written in a compiled language, and they fare poorly compared to commercial optimization packages.

If you can formulate your problem as an explicit system of equations and need a free solver, your best bet is probably IPOPT, as Aron said. Other free solvers can be found on the COIN-OR web site. To my knowledge, the nonlinear solvers do not have Python bindings provided by the developers; any bindings you find would be third-party. In order to obtain good solutions, you would also have to wrap any nonlinear, convex solver you found in appropriate stochastic global optimization heuristics, or in a deterministic global optimization algorithm such as branch-and-bound. Alternatively, you could use Bonmin or Couenne, both of which are deterministic non-convex optimization solvers that perform serviceably well compared to the state-of-the-art solver, BARON.

If you can purchase a commercial optimization solver, you might consider looking at the GAMS modeling language, which includes several nonlinear optimization solvers. Of particular mention are the interfaces to the solvers CONOPT, SNOPT, and BARON. (CONOPT and SNOPT are convex solvers.) A kludgey solution that I've used in the past is to use the Fortran (or Matlab) language bindings to GAMS to write a GAMS file and call GAMS from Fortran (or Matlab) to calculate the solution of an optimization problem. GAMS has Python language bindings, and a very responsive support staff willing to help out if there's any trouble. (Disclaimer: I have no affiliation with GAMS, but my lab does own a GAMS license.) The commercial solvers should be no worse than fmincon; in fact, I'd be surprised if they weren't a lot better. If your problems are sufficiently small in size, then you may not even need to purchase a GAMS license and licenses to solvers, because an evaluation copy of GAMS may be downloaded from their web site. Otherwise, you would probably want to decide which solvers to purchase in conjunction with a GAMS license. It's worth noting that BARON requires a mixed-integer linear programming solver, and that licenses for the two best mixed-integer linear programming solvers CPLEX and GUROBI are free for academics, so you might be able to get away with just purchasing the GAMS interfaces rather than the interfaces and the solver licenses, which can save you quite a bit of money.

This point bears repeating: for any of the deterministic non-convex optimization solvers I've mentioned above, you need to be able to formulate the model as an explicit set of equations. Otherwise, the non-convex optimization algorithms won't work, because all of them rely on symbolic analysis to construct convex relaxations for branch-and-bound-like algorithms.

UPDATE: One thought that hadn't occurred to me at first was that you could also call the Toolkit for Advanced Optimization (TAO) and PETSc using tao4py and petsc4py, which would have the potential added benefit of easier parallelization, and leveraging familiarity with PETSc and the ACTS tools.

UPDATE #2: Based on the additional information you mentioned, sequential quadratic programming (SQP) methods are going to be your best bet. SQP methods are generally considered more robust than interior point methods, but have the drawback of requiring dense linear solves. Since you care more about robustness than speed, SQP is going to be your best bet. I can't find a good SQP solver out there written in Python (and apparently, neither could Sven Leyffer at Argonne in this technical report). I'm guessing that the algorithms implemented in packages like SciPy and OpenOpt have the basic skeleton of some SQP algorithms implemented, but without the specialized heuristics that more advanced codes use to overcome convergence issues. You could try NLopt, written by Steven Johnson at MIT. I don't have high hopes for it because it doesn't have any reputation that I know of, but Steven Johnson is a brilliant guy who writes good software (after all, he did co-write FFTW). It does implement a version of SQP; if it's good software, let me know.

I was hoping that TAO would have something in the way of a constrained optimization solver, but it doesn't. You could certainly use what they have to build one up; they have a lot of the components there. As you pointed out, though, it'd be much more work for you to do that, and if you're going to that sort of trouble, you might as well be a TAO developer.

With that additional information, you are more likely to get better results calling GAMS from Python (if that's an option at all), or trying to patch up the IPOPT Python interface. Since IPOPT uses an interior point method, it won't be as robust, but maybe Andreas' implementation of an interior point method is considerably better than Matlab's implementation of SQP, in which case, you may not be sacrificing robustness at all. You'd have to run some case studies to know for sure.

You're already aware of the trick to reformulate the rational inequality constraints as polynomial inequality constraints (it's in your book); the reason this would help BARON and some other nonconvex solvers is that it can use term analysis to generate additional valid inequalities that it can use as cuts to improve and speed up solver convergence.

Excluding the GAMS Python bindings and the Python interface to IPOPT, the answer is no, there aren't any high quality nonlinear programming solvers for Python yet. Maybe @Dominique will change that with NLPy.

UPDATE #3: More wild stabs at finding a Python-based solver yielded PyGMO, which is a set of Python bindings to PaGMO, a C++ based global multiobjective optimization solver. Although it was created for multiobjective optimization, it can also be used to single objective nonlinear programming, and has Python interfaces to IPOPT and SNOPT, among other solvers. It was developed within the European Space Agency, so hopefully there's a community behind it. It was also released relatively recently (November 24, 2011).