Even though we're using Ubuntu for our server, the Mac UI is well beyond the UI that Ubuntu can offer. If I am able to get things done faster with the Mac machine given its superior UI, then it is worth the extra $$. Otherwise your direction makes sense. My partner also has the Dell Inspiron 9400 and is going to wipe MS off of it and go with Ubuntu in the near future. My problem is that I still need XP to run my Revit CAD app, and the Mac lets me do this without having to reboot - which is a big productivity gain.
I'm just wondering, what specifically about the Mac UI gives you significant productivity boosts?
Great deals(once you get all their coupons and upgrade your own memory) on developement strength machines and it's good to support initiatives that provide alternatives to MS.