Yes - really easy. You just 'deregister' and then re-register it under your wife's Amazon account details.
There are details about gifting Kindles here:-
http://www.amazon.co.uk/gp/help/cust...6580#givingnew
Which appears to indicate that if you mark the Kindle as a gift when you buy it, it won't come pre-registered, so you wouldn't have to de-register and re-register.