Same offer that was on until end of March. Been waiting for this to come back. It restarted on Friday 1/5/09 and ends on the 28th.
As always: basically if you transfer your account you get a £100 credit, provided you meet the not too difficult criteria. You must fund at least £500 per month to the account and you must use their transfer service to switch, and you must also have a direct debit to switch.