Today, I have released a new, considerably reworked, version of csipopt.
The original API is maintained, except that the Ipopt class has been renamed IpoptProblem. This class is no longer sealed; instead the API has been refactored to facilitate sub-classing of IpoptProblem.
I have managed to work away the unsafe directive requirement, yielding considerable more straightforward calls to and from the native Ipopt DLL. In fact, the code can now even be applied in Silverlight 5 applications under certain conditions.
Please feel free to use and comment upon this reworked version. My hope is that the performance and usage should have been substantially improved with this new version, but only through extensive usage and commenting this fact can be proven or true or false :-)
Good luck Ipoptimizing!