From what I gather, the reason they say to wait 3 months is because of the changes to the cevix (as Margot says). This means that you may get a false abnormal result.
The reason I had mine so soon after birth is because I was due a smear near the beginning of my pregnancy and was anxious to get it sorted asap.
S - JULY 10
Sibling to Sawyer due 6.12.12
Tight-Arse of the Year 2012