My question may be similar,
Using the new RDP client that comes with Vista and can download with XP here is what I think your issue and mine is,
When one connects the darn new RDP client puts in the host you are connecting to such as if you are connecting to remote.user.com, the stupid client puts in that address as the domain name!
How utterly lame is that, the appears to not be a place to enter in the domain name like before. So, of course the domain name is not going to be the same as the stinking host name or fdqn your connecting to.
The username entry would look like:
remote.user.com/administrator
password
Or
1.1.1.1/administrator
password
The logon fails of course, then you have to manually go up and clear out the false domain crap and correct the username, then logon is fine.
I am going to make a new post on this but wanted to add this so you get alerted if this is your issue.
Jason