I've had numerous people email in regarding the issue, also discussed here in the Nokia Discussion Forums. Essentially an 'old' update pops up as available, as shown below, but the version number makes no sense, being a significant downgrade from whatever's already on your phone. Plus, if you try and install it, the update never completes.
My advice then is to ignore the update notice - it'll go away eventually, when someone at Nokia works out what's gone wrong with its firmware OTA system.
And my award for optimist of the year goes to the reader who thought this might be Belle Feature Pack 3 arriving....(!)