Now playing: FFVII Crisis Core, LEGO Indiana Jones.
My wishlist: Elder Scrolls Travels, Jackson/Livingstone's Fighting Fantasy.
I am 99% certain that when you have the WLAN switch on then your PSP is actively looking for a connection to log on to - which uses more power than if you don't have it searching for anything at all. It can't hurt to turn it off if you're not using it...
On the PSP Slim and Lite at least, merely having the WLAN switch enabled is at least doing something. I'm not entirely certain if it's scannig for connections or doing something else, but I've been able to observe a definite and repeatable problem with loading from the UMD drive when the WLAN switch is on. Given that the problem can be eliminated simply by disabling the WLAN it's fair to assume that it does do at least something when enabled, even if it's only something very minor. I should probably contact Sony about my findings, as I believe it's a design flaw (or at least a long standing problem in the firmware) and may affect all Slim and Lite users. Whilst I don't believe the same problem affects the original PSP, I would imagine that it operates the WLAN in a similar way when the switch is enabled.
I've been intentionally vague in my reasons for assuming the WLAN draws power when enabled even if the game being played does not make use of it, but this is simply because I don't want to derail the thread. I'll leave my findings for another time and place.