To clarify: he is not the permanent host. Since 2002, Have I Got News For You has used strictly guest hosts. Shatner is the latest one.
Incidentally, my trip to London in January 2012 ended up being postponed to June, and I have tickets to see Have I Got News For You being taped. Alas, It's not Shatner's episode; the host that week will be Kirsty Young, former host of The People Versus.