I use the ingame option for quiting any game. And turning off the machine (in normal circumstances) via xmb.
But when NV crashes on the ps3 it locks out the user from the menu completely. The only option you have after waiting on the game to "unfreeze"... is to push the power to power down the console. This has happened to me numerous times with new vegas, and I have decided until they address this issue and I'm not going to play the game. As doing this will inevitably damage my system.
I have noticed that sometimes when I get a long pause and I get up to reset the system it actually does catch up again and keep moving lol Like I said I am lucky it has only happened 3 times but now I just wait for it to resume, seems alot safer than resetting and powering the system back on all the time! The thing is I explore everywhere I don't just follow the MQ, not sure if it has any impact what order you do things but as of now it is stable, hopefully the next patch they release helps with the Memory Leaks which I am attributing to the lockups after a stint of gaming, for me normally after an hour or so in one session, I will get the Memory Leak or Out of Memory Lock-up!
I also found when it happens when visiting a new area ALOT more often than when you fast travel or visit an area that has already been loaded, if you check your saved game file you will notice as you explore the size of the saved game gets larger and larger, this is because NV like FO3 loads the areas data into your saved game as you go! To help with this whenever I go to a new area I do a 360 degree spin, and move slowly to allow the PS3 to load the new area into your saved game, I have found if you run into a new area and it has NOT been properly loaded onto disk, that this is when you are most at risk of a hard-lockup!
Just my observations from playing!