My 4 year old son writes most of his numbers and alphabets in mirror image or reversed. Numbers 2, 3, 5, 6, 7, 9 are written the other way around. Alphabets like D, E, F, L, P, R, Z are also written backwards ie facing the other way. He also tends to write from right to left. He is left-handed.
I was beggining to get a little worried because I read that some children with dyslexia may reverse or transpose letters when writing. (I also read that dyslexia is not a visual problem that involves reading letters or words backwards or upside down, nor are such reversals a defining characteristic of dyslexia.) Still, I was worried.
Then I came across some forums and learned that its quite normal at this age. If your child does reverse or backwards writing and you are worried, you can check out the following links.
Five Year Old Writes Backwards
Writing in Reversed Letters
Updated 2014: These days you can help your child overcome backwards writing and letter reversals using apps like this one. LetterReflex App for iPad and iPhone.