I bought a used ipod from ebay :( it was stuck at the activate screen and seemed  like a simple thing to clear. About two weeks later and having great fun (?) downloading a ton of malware and spyware (always onto a sandboxed machine without access to my network!) decided to work it out myself. As it turns out it is really easy to remove a device from a previous appleID without knowing the existing account (one of the first steps I took was to find the existing appleID on the device,

which was easy - lots of youtube videos showing how to use an alternative DNS and crash and freak out your device to let you see the full email - unfortunatly someone appeared to have already restore/updated the device I have so this was not possible.)

Turns out you only need the serial number and a bit of clever code to remove it from the original appleid, although I've only so far tried this with the ipod I bought I suspect this will work with any apple device stuck at the activation screen.