Found this on the Adobe help blog
http://forums.adobe.com/thread/314870
You need to do this on the computer the Sony was authorised to (with digital editions running). Then re authorise on the computer you purchased the book on.
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Unfortunately there is no good way to tell the AdobeID that a Sony Reader was registered to. So the best alternative I can come up with, is to
Deauthorize your Reader in ADE (again this is seperate and distinct from the Sony store registration) by pressing "Ctrl+Shift+E" with the Sony Reader plugged in. This will remove any Adobe Authorization from the Sony Reader and should immediately ask you to reauthorize the Sony Reader. If it does not do so, with ADE still active, unplug and replug your Sony Reader from the PC should force the Device Activation dialog to come up.
If this doesn't work you best bet will be to open up a tech support case.
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Jim Lester
Adobe Systems
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Hope this helps