iMac with Core 2 Duo (not the 17-inch 1.83GHz model though)
Before you plunk down your $2 for this enabler make sure that your Mac doesn't already have 802.11n enabled (very recent Macs ship with 802.11n enabled). The product page for the Enabler has directions to find this out.
Airport Extreme 802.11n Enabler for Mac is available now for $1.99, and since it is a download you can get it within moments of paying for it.
Update: As has been pointed out (and which I took for granted), if you are planning on buying one of the new Airport Extreme base stations you need not buy this separately. It'll come with the base station.