Can I make eBuddy be the default instant messanger?
Normally you should just be able to do:
Menu->Settings->Applications->Manage Applications
Select the AT&amp;T Messenger App.
Then scroll down in the app Settings and there is a button labelled 'Clear defaults'.
Clicking this will clear any launch by default settings.

Next time you click the text icon on a contact it will ask you what App you want to use.
However maybe AT&amp;T have hacked the phone to prevent this.