gpuArray freezes/never returns

I am running Ubuntu 12.04 with a MATLAB R2012b installation and have CUDA 5.0 SDK installed. CUDA is correctly installed, which I made sure by compiling and running several of my CUDA projects on the computer. When I try to use CUDA in mex files or via the built-in gpu functionality, however, I hit a wall. In particular, every call to cudaMalloc or gpuArray freezes (possibly other cuda calls, didn't try). Did anyone encounter a similar problem and has found a solution?

[Edric Ellis]

The very first time you access GPU functionality, things can unfortunately take a while - how long did you wait? Also, which GPU device do you have?