Boxee was one of the first to do a USB wireless keyboard in a nice form factor, though it is primarily a remote. They are tough to find, especially the 'sold separately' edition with the USB dongle (instead of de-soldering the receiver from a Boxee). Here's one on eBay: http://www.ebay.com/itm/111569863840

There are a number of knock-offs now, such as http://www.amazon.com/dp/B0092KDSQW/

However, if the AdaFruit 'smallest wireless USB keyboard available' didn't work out then these probably won't either.