I second the other mama's advice on this one. All babies do different things at different times.
Are there any other developmental milestones that your baby HAS made on time? If so, then I wouldn't worry about this one too much quite yet. Also, if your baby was born early, that slows things down some too. Work that tummy time with toys. Try to find ways to make it a game for baby. Before you know it, baby will figure out how to roll over to get out of tummy time and will be moving onto other developments.
My baby was born two months early and she didn't really get the hang of the rolling over stuff until 8 months or so. Another thing...we are talking about a couple of months of behindness, not years of behindness, so really there is no reason to panic unless doctor is concerned. And believe me, your doctor has seen the gamut, he/she will know if you should start to look into intervention, etc.
Overnight my baby started doing everything we were late on. Rolling over, crawling, babbling, teeth, etc.
Hope this helps ease your mind a bit.