We were using source offsite without problems. After switching to source offsite collab, I have no trouble working from the Collab EXE, but when attempting to check-out files through VS.NET, I am prompted for my SOC username and password, followed by my VSS username and password. These are different, but regardless of what I enter in the VSS prompt, I am rejected. Is this a bug? My partner isn't having this problem.

My username and password must be cached through Collab, because I am no longer prompted when accessing the repository directly from Collab.

Did this ever work properly? Do you have a valid username and password in the VSS database?

I would suggest the following:

In the SOS Collab Windows Client, under Tools->Options choose to "Clear VSS associations."

Then try to open a project from Source Control using VS.NET. You should get prompted to enter your username and password. Does this succeed?