One thing that might help.
When you transfered the rented movie to your iphone, was it from the same library that you transfered the music from?
The iphone is set to only sync media from one library at a time.
It does not work like and ipod where you can set it to manually manage and sync between multiple computers or libraries.
If it was the same library, connect your iphone to the computer and see if itunes notices a large amount of other on your iphone.
If so, these following articles from the apple support website that might help out.
http://support.apple.com/kb/HT1737
http://support.apple.com/kb/HT1414
http://support.apple.com/kb/TS1275
You might need to reset and or restore the iphone to resolve any software issues.
Hope this helps.
