We basically want the forums to be self selecting; try and treat people like adults and hopefully they'll respond in kind. It's up to you what your avatar is, as most have said if there was an influx of more explicit ones then we'd have to make a rule which ultimately I'd see as a failiure. I think I've PMd about 3 people in all my time about avatars.
I'd say if you had a kid over the age of about 12, and he saw your avatar would you be bothered. If you don't really care - carry on, although I think Russ's about the limit I'd personally allow. Plus, it's a profesional image thing, if someone is the representative of a company or organisation, that all reflects back on them as mine would on PGi.
With Russ's, I've always been more suprised with the avatar because of his position with the PA and things, rather than the image itself, but it's a personal opinion rather than a "site" one.