I like this graph comparing Google searches for "Arduino" with "Basic stamp": http://www.google.com/trends?q=arduino%2C+"basic+stamp"&ctab=0&geo=all&date=...
Thanks Andrew,
just got back from the holidays and saw your post. Didn't know that Google is tracking every search term ... do we have to worry ... big Google is watching us 
Also thanks to retrolefty:
Now how to get a handle on user population and number of processor boards sold? That's kind of hard to get I would think. The member population of this forum is one metric, but don't know how inclusive it would be. There are so many manufactures now that I'm sure getting them all to give you units shipped to date would be difficult, but maybe worth some e-mails. A quick look on E-bay using Arduino as a search word will show a growing number of listings over time. Of course goggle can give you a count of site hits using a Arduino search word.
I've been writing to some distributors and manufacturers but unfortunately did not get a lot of positive response until now. Most treat their sales volumes as confidential and are not willing to share it, not even for academic use.