I currently have an older Dell notebook running linux (originally came with Win2K).
If you are just going to use it for work, assuming little graphics, It's a tossup.
But if you are going to use it for personal stuff also the Mac is really, really nice.
What sold me was a trip to the Apple store, then walking out and down the mall to the Dell kiosk.
One look at the Dells displays compared to the Mac sold me.
Mac
You can get some thinkpads w/ linux installed right from the OEM AFAIK.