Yes you can transfer the rentals. I had downloaded a few rentals and before I could watch them there was an update available for the ipad. I had already transferred them from my mac to my ipad so was concerned that I would lose them if I couldn't transfer them back to the mac before updating the ipad.
So I searched the net and found many people saying it wasn't possible to transfer from the ipad back to the mac.
Well you can and this is how to do it:
1. In iTunes select the iPad
2. Have a look at the top bar eg Summary, Info, Apps etc. Select "movies".
3. This will bring up a page which has two sections: "Rented movies" on the left and "On (ipad's name)"
4. You will notice that the movies show up in the section "On (device name)" and they have tabs which include one called "move".
5. Select "move" and it will transfer your rental back to iTunes on your mac in a few minutes (depends on how large the file is of course)
6. You can now update your iPad in comfort and re-transfer the rentals once the update is complete.
7. Enjoy your movies
I hope this helps ;-)