Is it HFS, HFS+, HFSX? Is it the same in ipods, ipads and iphones, and across different versions of iOS?
It's HFSX.
According to "Mac OS X, IPod, and IPhone Forensic Analysis DVD Toolkit" book, the IPhone Partitioning scheme (which is quite similar to similar to Apple TV) contains:
1 - A first Master Boot Record partition (around 500mb),
2 - followed by an Apple_Free area,
3 - then a a first HFSX partition, which primarily stores the iPhone's operating system.
4 - Again, another Apple_Free area
5 - and finally the second HFSX partition holding all user data (apps, movies, pictures, whatever)