I searched forever for this too and never found one. Until about a day ago. It was first posted on the Yaahoo Sony U group, and I reposted it on a few Sony U forums (handtops and jkontherun), I'll repost my message here:
"As alcuin2k (nice find man!) posted on the Sony U Yahoo Group, he's found a small USB thumb keyboard that will work with the Sony U. It's actually designed for the PS2, but works with WinXP with no additional drivers.
Lots of pics in this thread: http://www.mp3car.com/vbulletin/showthread.php?t=41136
The guy who bought it got it from EB Games, who is out of stock online:
So you'll have to go to an actual EB Games in your local mall to see if it's in stock there. However, it's also sold online here for $1 more than from EB:
http://www.gameexpress.com/product_deta â€¦ 7742005709
Obviously since it's not designed for computers it's missing a few comforts and keys we might otherwise want (control, alt, etc) but the mp3car thread's very useful if you get one. It's got a key remapping utility posted, so you can remap the higher F buttons you never use (one guy did F6->Win-Key, F7->Ctrl, F8->Alt, F9->Del,F10->Escape).
Maybe not the best or the way we'd have designed it but with not many other options available, and for only $22 including shipping, it's well worth it IMO!"