Both my PS3 and Sony blu-ray player have the 24p/24hz setting enabled and blu-rays play fine and work at 24p/24hz, it's just other video content does not - even though the frame rate is 23.97fps!

My TV is a year old Panasonic GT30 Plasma, which has no problem doing 24Hz playback on blu-ray movies.

Taking all things into account, someone suggested to me the file structure of the AVCHD, so I'm assuming maybe it is not reading it right and defaulting to 60Hz? I'm going to try and play with the file structure of AVCHD and see if that changes anything - maybe I'm not doing it right!