I recently installed this app and it is the best text messaging app for the G-Slate i've found so far. It syncs your messages with Dropbox then sends the message through your existing phone number on your primary Android phone. You are able to see any text messages received on your Android phone as well as your sent messages.
For more info here is the market link....
SyncSMS
SyncSMS lets you sync your text messages between your Android phone and 3G or WiFi-only Android tablet. This app lets you receive text messages on your tablet which have been received by or sent from your phone. Also, this app lets you compose text messages on your tablet which will be copied to your phone and sent out by your phone. You will need to sign up for a free Dropbox account, if you don't already have one, in order to temporarily store the messages between syncs.
For more info here is the market link....
SyncSMS