Not sure which operating systems this will apply to, maybe only Vista and Windows 7, but the SMS Lobby software wants/needs to run with elevated privileges, ie. administrator privileges.
When you start your Teamspeak, you area most likely doing so with only your user-level privileges.
The SMS Lobby starts the nr2003.exe, so it is also running under elevated privileges, and the lower level Teamspeak cannot steal input from another process that is running with higher level privileges, thus your push to talk button does not work while in-game.
If you right-click Teamspeak, and "run as administrator", it will run with the same privilege level as the SMS Lobby and NR2003, so your push to talk button should start working again.
