Will the big G penalize me if I display certain content for the United States, and different content for the rest of the world? The differences between the two versions would be minimal. In one way I would think this they might consider this cloaking, yet, it's a very useful thing to be able to do. What would happen if US-based Googlebots compared their cached version with UK-based Googlebots? Would they collaborate and interpret the difference as merely updated content, or would they think "Hmm, something weird is going on here... since we are simple bots and don't know any better, let's drop their rank a few hundred positions". Thoughts?
As far as I know google has only US-based Googlebots. I've never seen a Googlebot coming from an .uk IP, at least not a real googlebot.
No they won't as long as you show the same content to the google bot as you're showing to your users. Matt Cutts discusses that very thing in one the videos on this page, I forget which one it is though. They're all worth watching though. http://video.google.com