the touch doesn't come with any form of virus scanner or firewall, it just have a wifi connection and safari, not that its likely but if the touch did manage to get a virus you can just as easily restore the touch and start over
I have an iPod touch and don't worry about viruses. The iPod touch runs Mac OS X and as such at the last count there are no viruses for this. Also Safari, the Mac browser, is pretty secure too.
I guess really you should follow the usual precautions.
One thought though, there ought to be some sort of parental control - if a kiddie gets one for Christmas and parents are oblivious to the fact that this has the 99% really-for-real t'internet on it.