This was my thought at first, but why would Apple have a USB adapter? People are already testing USB headphones, keyboards, etc.
THe answer is money, two-fold.
1.) Apple will make money off adapter sales for the current iPad.
2.) And again when people upgrade to iPad v.2 when it includes a built in USB port, SD slot and for the LOVE OF G@D a CAMERA!!!!
The functionality people are testing is unsupported. Apple have always been clever about having unsupported functionality in their devices that they could disclaim responsibility for and keep unadvertised, but would spread by word of mouth and contribute to the perceived functionality of a device.