Ditto PP. Every baby is different and this isn't something to be concerned about at this point. FWIW, DS rolled over front-to-back at 1 month and back-to-front at 5 months. If your pedi is not concerned don't worry about it.
DD was rolling over around 4 - 4 1/2 months. But recently on the April board I read a few people celebrating that their 6 months old had just started rolling over. There were a few people so I think it is totally normal for it to be up to 7 months. Like pp said, if the pedi isn't concerned you shouldn't either.
Re: roll over?