I'll preface this by saying sometimes, I think I know too much for my own good. I'm an occupational therapist and I get super paranoid about development with dd.
DD is 5 months old and hasn't rolled over yet. Actually, she shows absolutely no interest in rolling over. The only time she's even come close is when she was trying to suck her thumb. That's about the only thing she's interested in. She's met all of her milestones on the late end of "normal" but I don't see her rolling any time soon. When should I really start to worry about her not reaching gross motor milestones?
Re: When to become concerned about milestones?
My 34 weeker just learned how to roll over a couple weeks ago, just shy of 8 months actual. I, too, was worried. But, he always manages to surprise me when things "click". Now, he's sitting up unassisted (although very wobbly) and is trying to army crawl.
Don't worry - she'll get there!