this actually wouldn't be too difficult, if you wanted to do it in a loose fashion...
1. Guests browse via a unique session, so just allow X page views per guest session... This would be the quickest, fastest, easiest way, however it would mean if they came back later they could browse X more pages. (But I think most would get the hint the first time they encounter the Please Register page).
2. Bots/spiders are tracked and recognized in vB3, so just exclude them from the limitation.
Okay, but now if you didn't want to do the simple session method, then you would need to start tracking via a cookie and/or IP method, which, again isn't all that difficult, but it could build up to a large amount of data depending on how many guests browse your forum. You would need to purge off old data after a certain amount of time.
In the long run, I find that people sign up for my forum simply because of the amount of good information they find, and they would like to ask their own questions and such. Also, I have viewing/downloading attachments disabled for guests, so that is also a little encourgement.