The change comes with a new developer API that will actually make things much easier for other developers, who've had to rely on a few undocumented calls up until now. Current licensing agreements with labels—who Last.fm is in no position to alienate—prohibit mobile streaming, though the company's official mobile radio apps—right now just on the iPhone and Android—will still work fine.
As you could imagine, this kind of blows for a lot of people. Windows Mobile users will no longer be able to use Pocket Scrobbler, Symbian folks will have their beautiful baby, Mobbler, ripped from their hands, and BlackBerry owners will soon find FlipSide, a pay app, rendered silent. And as much as I'd like to, I don't really believe that we'll see official clients for any of the platforms, at least not soon. [Last.fm—Thanks, Jealousy!]