Multiple remote logins from Windows clients to a Mac mini server
With Lion this is possible, but I think there is a limit of 2 sessions, and you don't need to be running the server tools.  When you screen share a Mac that has it enabled, you can choose to either just take over the screen as per the scenario you are used to, or move to the "virtual display", where you can log in as a 2nd user.

Many (particularly PC/Windows) VNC clients are getting tripped up by this new behaviour and do not connect at at all at present, so you might need to careful with your choice of client.

Edit: I think the 2 session limit is to provide the ability to allow an administrator to login on the 2nd session to work without interupting the normal user, I would test whether more than 2 sessions are available, but I can only get it to work from a Mac client using screen share via the finder rather than a specific VNC client app, and I only have 2 Macs...