Have iPeng + iPeng HD. They work well as controller UI's. Squeezecast for iPhone, iPod touch, and iPad on the iTunes App Store works well as a player you can run in the background (which then means the iTouch / Pico Slim / JH13's combo is a mobile streaming client within wi-fi range of your server)

Squeezey for iPad on the iTunes App Store is another client that's been mentioned. The Author of iPeng has hinted at possible integrated playback at some unspecified point in the future. When its ready.

Thanks much Grahame. Between this and FLAC Player, things open up a bit with playback in iOS.

As a follow-up, ended up picking up iPeng for the iPad, and after a few drinks, Squeezepad also. Lived with both for a few weeks and have to say it comes down to more features (iPeng) versus better organized UI (Squeezepad). As I find myself rarely using the extra features (open up browser when I need to), I pretty consistently drift towards the much more Apple Remote-like Squeezepad. Keeping the iPad pretty. If you want everything from the remote or do lots of playlist organization, look at iPeng.

Also the Squeezepad in app streaming functionality seems to work well for files up to 16-24 48 (for B&Ws Society of Sound stuff fine), but over (HDtracks 24-bits, HRx, etc.) doesn't stream (or convert). iPad gonna limit USB out too obviously.