If you hadn't needed to pull the trigger for tax reasons (same here), I'd say to wait for the Nehalem MBP next year, but if you need it now, go iMac. Use something cheap for the road - the increased screen size and speed when you're at home will more than make up for not having the the Mac on the road, and you can always buy yourself a low end MacBook if you later feel you need a Mac for trips too.
Of course, this equation could change if you spend a large proportion of your time on the road, and you didn't say that. (I don't anymore, so this tilted it for me.)