Greg's answer is correct, however you'll find that most panels have little bump outs where the mounting screws go that just happen to protrude about 1/4 of an inch.
If your panel is like this, you should be able to mount it on concrete. However, if the poured wall isn't all that smooth, or the forms weren't well aligned, you may have concrete touching the back of the panel in places. Usually, this is right where you want a screw to go such as for a grounding bar or the main bonding jumper.
I'd suggest using some washers or short slices of 1/2" PCV conduit to make spacers to get the panel well over 1/4" off the concrete unless its a real smooth wall.
