WARNING: The below tip will kick out one of the already logged in users. This behavior may have been added after I came up with this advice, or I've always been the evil person on the block :D
Even though there are no way to connect using the standard remote desktop program you can still "hack" a connection. Sure the limitation exists, probably to sell more licenses or to protect the host server from getting too many connections, but to get past it you do the following:
mstsc /v:myhost.com /F /console
Where myhost.com is the name or ipnumber of the server you are trying to remote to. /F means a full screen connection, /console means to connect to the "console session of the server" (whatever that means, it anyhow results in you getting in although the server would otherwise refuse you).
The full format of the MSTSC command call are:
MSTSC [<Connection File>] [/v:<server[:port]>] [/console] [/f[ullscreen]]
<Connection File> refers to an rdp file to be used with the connection (good if you need to make local drivers or other resources available or set up the connection otherwise).
You can also call MSTSC with the "/edit" switch if you wish to edit a connection file:
MSTSC /edit <Connection File>
Finally you may also migrate files by using the "/migrate" switch (not 100% sure how this is done though since I don't have an older version file to test with).