Aside from Costco beating out Apple by $20, I think it's a little strange that the iMac is the only machine they seem to carry. A search at their website for "mac" returns nothing but a few software packages (including Quicken 2006 for Mac for $60 with a $20 rebate), but "imac" actually returns this one lone model they're selling. There's no word on how long they'll be carrying these machines, but with the rate that PowerPC based machines are disappearing, I'd recommend you pick one up while they're hot if you're interested.
[UPDATE: Wow, this price is even better than I thought. Turns out that $1479 includes a 3-year AppleCare program. Run, don't walk! Thanks Derek!]