I am reluctant to post on this as I think I know the answer, but DP is quite concerned about it so said I would seek the wisdom of mumsnet!
DD is 3.10 and loves drawing and writing. She can write her own name with no problem, and any other word (with varying degrees of legibility!) if it is spelt out to her or we write it for her to copy. However with letters other than in her name and with numbers, they are very often written backwards.
I never try and correct her, as past attempts when she first took an interest in writing did not go down well! My assumption is that she is pretty young to be writing as well as she is (the couple of friends I have mentioned it to have said the same), and with time and practice this will probably correct itself.
However DP wonders if by ignoring it we are potentially making it harder to correct later, or whether might be a sign of something more worrying. I do intend to ask at her pre-school parent's evening too, but does anyone here have any experience in the meantime?