I've used Twitter on a few occasions using VieraCast, but I usually have a laptop, desktop, or my Droid handy, and that beats having to grab the Wireless USB keyboard (which does make it a LOT easier to type than using a remote) and Tweet through the VT(weet)25 :)
I am enjoying all the new capabilities that they keep adding to VieraCast though. The one thing that I still can't quite wrap my brain around is the German Entertainment news channel ...
Also, remember you can change your VieraCast home screen by going into the Settings (press 'More' a few times) , and that will allow you to move the icons/apps around on your home screen.