There are a lot of options left out of the poll!
I am happy with my MBP, so I guess my trigger would be getting it stolen, lost, or damaged in a way not covered by AppleCare.
Also, I've been thinking about a Mini media server setup, so an improved Mini (better graphics processor, and/or cheaper price) might tempt me...
On the other hand, do I really need a media server? No, not really. To me it's just a cool idea, and I really have better uses for the hundreds of $ it would cost me.