I went through the same decision last month. For me, the laptop portability isn't crucial as I don't NEED to take my computer with me. So it'd spend 90% of it's time at home hooked up to a display, keyboard and real mouse... So what's the point really? For that one trip I can either live w/o or buy a cheap XP based lappy. The fact the iMac is faster, cheaper and has a bigger monitor (since I was looking at 20" ACDs) pushed me that way. Plus, with it in our living room, the iMac looks cleaner than a lappy w/ACD would.
Basically if you'll take the laptop with you, get it. But if not, don't waste the money.