OMG! Ubuntu! posted a guide about using in the Ubuntu font in gwibber
If you're on Maverick (or even if you're not) chances are you're in love with the Ubuntu font and as such want to use it everywhere you possibly can.How to get gwibber to use the Ubuntu font
Gwibber, when using the 'Ubuntu' theme, sadly has ideas of its own but enabling Ubuntu-font usage is no more than a minor-hack away.