Same problem here, on a 2 months old iMac C2D, except that I never installed LW on it before. It works on my mother's Mac Mini, though. Same install, very similar computer...

I get a blank window before even being able to configure anything. I can only see "Cancel" at the bottom right corner.

Trying the beta 4.13 right now. It suggested nothing but an upgrade, whereas I just deleted Limewire. It hung and I had to forcequit it. Bummer.

Re-launched the 4.13 install, it worked. Launched Limewire and I get the same blank window with only "Cancel" in the corner.