I seem to remember someone asking a similar question - they wanted the Messaging tab to route to WM messaging instead of the TP2 messaging. I just tried to do a search and didn't find it, but it's probably a couple of days back in the forum - I'll let you know if I find it!
EDIT!
So I found the thread I was reading. Someone coincidentally JUST posted to it.
http://forum.ppcgeeks.com/showthread.php?t=114890
It's not an exact fix for your problem - but there's some registry stuff toward the last page that might help you start on your voyage to fixing your tab. Seems like you'd have to edit something in the HKLM\Software\HTC area of the registry. I don't know though - but now you're one small step closer!