reguardless of how small the update is, or may be... I'd cancel it. At the very least- you'd be able to get the same iBook you ordered today for a lot less.
Not to mention- reguardless if it's a big or small update- it's still an update...and if there is one- the second it happens, your 1 week old iBook is suddenly a used year older. If you never plan to sell it to upgrade in the future, that's fine... but if you do, that's another consideration.
Before I cancelled though, I'd check the estimated shipping time... you may be lucky enough to get it upgraded for free

(again, this is all assuming the updates actually come this time....)