Back of the strategy guide says it reveals all locations for hallowed out rocks but where/what are they? I was just looking at the guide at work but it was wrapped in plastic so I couldn't check it out...

There is a hollowed out rock in the hidden valley, look for one with some red graffiti on it. Just had a few bits and bobs stashed away.

There's one between the Mojave Outpost and the town that the Legion overran (name escapes me at the moment). Just go downhill form the outpost and hug the wall. There's a plane crashed in the desert and above that is a cave (the name of which also escapes me). Right in front of the rock is a hallowed out rock.

Oh so they act like containers?

Yeah and they usually just have random crap in em, a few 20g slugs, a scrap metal, ect.

Bunch of hollowed out rocks in Hidden Valley. They are all marked with graffiti though. Those are the only ones I've run across aside from one outside of some dungeon/cave I can't remember where.

There's also one at the Yangtze Memorial. The rock has a peace sign on it.

There's one right in front of you if you fast travel to Jacobstown (if not, go to the front gate, it's directly in front of it).

Yes, just like the hallowed out rock behind megaton.

Except that one had good stuff in it