Yes, it can cause problems for people at work, that's true. I don't know what forums you hang out in Nathan, but VitalMX is a pretty out there place for personal abuse, criticism, bad language, nudity, sexual innuendo, racism, you name it! Beats me how they don't get sued to Hell and back with some of the slander on there. Now personally, I like that - I love the sort of free-wheeling nature of the place, but I do realise that many people have a different perspective.
As far as OzVMX goes, I am inclined to be OK with nudity in avatars, so long as it's no more than we'd see on TV or in a typical magazine. However, really 'rude' images or sexual innuendo is probably a step too far on the whole. And I think any personal byline that suggests anything about another forum member is No Go.
So, here's my proposal for personal sig/avatar guidelines:
Generally tasteful
Relevant or humourous
Nudity OK as long as not explicit
NO personal insults/criticisms or sexually explicit/racist bylines in personal sigs.
And Firko, as far as I am concerned the Kevlar girlie is absolutely fine