Yeah, pretty sure everyone sort of expected that Raiden would be part of the PlayStation All-Stars line up. In fact, when Sony started talking up the fact that third party characters would be involved, a samurai sword wielding Raiden was the first character I thought of. On its own, this might not be massive news -- but it's timed perfectly with the fact Sony are now accepting application for the PlayStation All-Stars beta.
The beta takes place from September 11 to September 18, and you can apply here.
Plenty have cried foul over the PlayStation All-Stars game's very existence, citing plagiarism of Super Smash Bros. It's a fair call, but at the same time, I really enjoyed the time I spent with the game. Despite its many (many) similarities, PlayStation All-Stars has a few points of difference, and it's just a lot of fun in general.
As for Raiden's inclusion, well it looks as though he plays exactly as you might expect him too -- with the sword slashing and the juggling and what not.