Update: This has been fixed in Ubuntu 9.04. Excellent!

tsclient is a Microsoft Remote Desktop work-a-like for Linux only better (it supports VNC, RDP, XDMCP & ICA)

The bug:
On Ubuntu versions greater than 8.04 it has a _really_ annoying bug. I call it the "tsclient locks screen while attempting an RDP session" bug.

How to reproduce it:
Basically if you open tsclient, type in an IP address / hostname into the Computer: field and press enter to connect. The result is your mouse ceases to function and you can't get control of your mouse and keyboard until you either press escape to get out of the tsclient RDP season or CTRL + ALT + F1 to get a console and then kill tsclient from the command line.

What you see
You know you're in trouble because a graphical element from the tsclient "Computer:" drop down list appears in front of your RDP session as can be seen in this picture.

The work-a-round:
Open tsclient
Type the IP/Hostname into the "Computer:" field
Make sure you click into any of the other fields
Then press enter to connect or click connect.