Hi Thomas,
The only thing I would suggest are the following:
First, make sure iTunes is up to date. Then run an update on your Nano. Even in the event it is updated, then run the update again...just to be sure:)
Next, I would eject the Nano, close iTunes and restart your Mac...many a miracle has happened by doing this. The computer gets to take some time to 'digest' the new updates 🙂
If all goes to according to plan (cross your fingers), you should see your rental in iTunes under the Rent tab. You should be able to drag and drop it on to your Nano. The rented movie should download to your Nano with the familiar 'download' wheel a turnin, along with the status in the iTunes status window (at the top of the window).
One more thing. I don't use synch. Things get a little out of control for me when I have the synching on. I'd rather 'manually' drag and drop. I can see what's going on. With that said, I too had to take a couple of takes before I got the "Italian Job" on my new 80GB Classic. I can't answer the question if there is a tab for the rented movies for synching. Remember this is brand new territory for everyone, I find it helpful to "expect" a few glitches. For example, last night when I was watching the "Italian Job" the movie faded and cut out at 2'20", right after all the intro credits. So I just fast forward to 3' and had no problems from there. I suspect when I was downloading it I was fooling around on iTunes and may have caused a hiccup in the download process. Bottom line...I'm really happy with this. Sorry for the long rant. Hope you get things going. Keep checking out this forum and Apple support...you'll find your answer. Good luck.