Very strange.... I got it to work in a command prompt window running poclbm at the command line exactly ONCE today with Slush's pool.  Got 3 or 4 shares over several minutes.  Have tried both poclbm and Guiminer several times with both Slush's pool and BTC Guild with no success.  No shares accrued

In either case, the poclbm output reads:  8 MH/s (~0 MH/s) [Rejected: 0/0 (0.00%)]