The folks at The Behemoth had to recode Castle Crashers for the PS3, so they decided to add in a bunch of cool new stuff while they were at it. For instance, matches in Play Arena will let you choose teams now, going 2-on-2 or full-on badass with 1-on-3.
The Behemoth has finished internal testing and has sent off Castle Crashers to a third-party to do some final testing. This means that you're going to see the game on the PlayStation Store pretty soon. They're not prepared to drop a solid release date yet, but they do say it's in the "final stages." Also, there's no set price yet.
And, according to the comments section of a PlayStation Blog post on the update, there will be a demo of Castle Crashers on PSN. The demo will not include the Arena mode, though.
Castle Crashers PSN Update: New Features [PlayStation Blog]