This seems to be a new thing with more recent dev builds - though I haven't worked out exactly with which version it started happening. Pretty sure it was after the last 'stable' release.
Also, it's iflash modded ipods, so that might be a factor (I could experiment with a hard-drive based one, but haven't yet).
The setting whereby you set the length of time before back light turns off after button presses, seems to get 'forgotten' after a while.
It's odd, in that it works fine for a while immediately after you set the time delay from the menu. But at some point during subsequent use it starts forgetting to do it, leaving the backlight on indefinitely, until you go back to that menu and set the time limit again, whereupon it starts working as it should again.
It doesn't seem to be that it has to turn off and on again in order to start forgetting. I'm not really sure what the action is that makes it start doing that...OK, just tested it and, for example, playing rockblox seems to cause it to forget to subsequently, once you quit that game, to turn off the backlight until you go back and set the time limit in the menu again.
It is of course possible it's an iFlash issue - if that setting is written to disk perhaps it gets corrupted and then read back in a way to mean 'turn off backlight NEVER'? But it didn't use to be a problem with older releases of rockbox even on the same ipods.