I have back dimples, though i've only just noticed =/. I can imagine them only being visible if you're not too thin though. I'm a size 8/10, and i don't think i had them when i was just a size 8.
Doing weights may put some (i wanna say substance?) on your back, so the dimples will be visible.
I think they're a good thing - I like a shapely lower back (provided, naturally, that it sits above a nice firm booty). A lower body fat will of course help you out and I imagine that deadlifting for a stronger lower back can't hurt your definition in that area.
EDIT: God dammit I hate it when I reply to thread rezzers without realising.
be careful i herd that if u sweat alot then the sweat can go into the back dimples an dry up causin a mouldy smelly back dimple to fill up with dried sweat which gets rid of the back dimple and gives u aids