If you want historical figures, then
this site is good, and
this one shows data for all types of computers (some fairly depressing graphs there if you're a fan of diversity, though.
As for more current figures, someone fairly recently did a quick poll. Before that, I've been working on an "Amiga Counter" similar to the
Linux Counter - the idea being that you'll be able to track changes in the userbase over time (including detailed hardware specifications). Unfortunately I haven't been able to contact the person who volunteered to help me with the php for over a month - if anyone would like to finish off a simple script, then contact me.
