The problem with this is the same as for services like dropbox. If enough people do this they will start blocking the slicehost IPs.

To be successful you need to have * IP usage distributed over a wide range * private distribution of proxy details * ability to switch to a new proxy when your current one is blocked

Tor tries to accomplish most of this, but from the OP's post it appears that it is being effectively blocked, either by watching Tor entry points or by packet inspection.