I had a similar problem after upgrading to Snow Leopard even though I had an up-to-date working version of Parallels 4 before the update. I found that somewhere along the way I had lost read permission on /usr/local. Make sure you have permission to read /usr/local/lib/libprl_sdk.dylib and manually fix any directory permissions that aren't right. In my case chmod o+rx /usr/local fixed the problem.
Hope this helps!