Sidekick and Helio UIs. Although Palm has confirmed his departure from the company, they aren't giving up the goods on where he's headed next, though John Paczkowski over at D says "multiple sources" indicate he's on his way to Google, presumably to work on Android.
While Matias was unquestionably a prime mover at Palm, we had heard rumblings that his rigid attitude toward design and control-freak nature sometimes caused conflict amongst the team. Of course, you don't crank out something as groundbreaking as webOS without some hard boundaries. The news that he's headed to Google could signal a major shift for Android towards dealing with the myriad UI issues the mobile OS has, particularly a lack of cohesion in the stock builds -- something HTC has gone to great lengths to shore up with Sense.
Update: And it's confirmed. Matias will be joining Sidekick pal Andy Rubin over at Google as Android's User Experience Director. Watch out HTC... and everyone else for that matter.