I've found another working Wifi USB dongle, a bit by accident!
I was given a non working dongle that a friend of mine bought for his PAW which claimed to be the correct chipset but wasn't. I thought I'd give it a go on my old Pi B to see what it did. Sure enough, it didn't work on that either. But laying on my coffee table was an old Belkin Wireless G USB dongle. I'd brought it out as I was building a new desktop PC and wanted a quick way to connect to the internet before installing a proper card.
Anyway, I thought no harm in trying, there's probably no chance of it working, but let's have a look. I plugged it into the Pi, plugged my monitor in and booted it up. To my surprise the boot screen got past the point of failure and appeared to set up a Wifi hotspot. I got my tablet out and now I'm happily connected to it!
It's the Belkin F5D7050. It's a bit large but hey, this is experimental anyway!
http://www.belkin.com/uk/support-product?pid=01t80000001qJZtAAMDoing a bit more digging, this dongle was available with several different chipsets.
http://linux-wless.passys.nl/query_part.php?brandname=BelkinI've got two revisions of this dongle and they both work, with the Intersil ISL3887 chipset and also the Ralink RA2571W chipset.