Poll added. I use 10.5. Wouldn't be an issue for me.
10.4 or earlier
Not that i'm trying to suggest that I might actually work on something or anything useful like that, but I was looking at some of the new Cocoa libraries in 10.5 and got to wondering what it's adoption rate among us is?
Personally i'm still running 10.4 on my Mini since it works and I haven't seen anything inherent in 10.5 (e.g. faster, more stable, etc..) to make me want to upgrade. Though there are 10.5 features i'd like to see my FE of choice utilize, but seeing as how that developer is lazy, useless, and lacking motivation, I won't hold my breath. If only I knew where he lived and could get my hands on him...
The follow-up to those that are still pre-10.5 would be, what would your reaction be to your FE of choice starting to require 10.5 and leaving you in the cold?
my mini has 10.5 installed, however that doesnt do me much good with a blown logic board