Our DD has one. The creases on the back of her legs didn't line up either. We had an u/s to check out her hips and the GP isn't concerned about her dimple.
THe mat health nurse started freaking out about both and mentioned spina bifida and and and.....I'm really glad my GP is a bit more relaxed about it all.
This was after I had a high risk down's pregnancy (1:24 risk) and the bowel was echogenic in the 16 week scan we had, so off to see the genetic counsellor to rule out cystic fibrosis. OMG...having babies is really scary!!!