I've looked down at the 'who's online' thing at the bottom of the page several times now and seen Google [bot] in purple. What does that mean?

Simply that Google is crawling the site. It helps people find us. No problem (that I am aware of).

When I saw it, I wondered if it was a spambot, myself. I didn't think to ask about it.
Of course, if it was a spambot, I think it would have posted stuff by now :P

(There was spambots on another site I was on, posting links in the shoutbox... so annoying...)

Yes that is annoying. It is not, though, from what I can tell.