What sort of strange UA is this?

[mikomido]

Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.6) G

[rocknbil]

Looks like FireFox on an NT based OS to me, you're missing the part after "G":

Mozilla/5.0 (Windows; U; Windows NT 5.2; pt-BR; rv:1.8.1.6) Gecko/20070725 Firefox/2.0.0.6

[mikomido]

"you're missing the part after "G":"

Well, that's the thing... it shows up like that in my log.

[GaryK]

I see this sort of thing in my logs all the time. It's far too easy for people to edit or spoof their user agent. Your example looks less like a spoof than a screw-up to me.