Have I missed something? I've been re-reading the first 6 books (yes, I know, I'm cutting it close...I'm still on book 5!), and I've not yet come across a reference to what happened to Harry Potter's grandparents (or any of the other wizarding kids' grandparents besides Neville's).
Sirius, in OOTP, mentions that he spent the summer holidays with the Potters, but no one ever says how they die. I imagine they must've died before Harry was sent to live with the Dursley's, but then, we never meet Dursley's grandparents either, just his Aunt Marge. In the Mirror of Erised, Rowling doesn't specifically mention grandparents, just at least 10 other people with "other pairs of green eyes like his, other noses like his, even a little old man who looked as though he had Harry's knobbly knees..."
Are we to assume that most of that generation died out due to Lord Voldemort's reign of terror (including the muggles)?
I suppose being a new mom has got me thinking about this sort of thing, since, as of later today, we will have received visits from both sets of Jacob's grandparents in one week (yay!). I guess we'll have to wait for book 7 to see if we get any mention of Harry's. Perhaps Harry will visit the elder Potters' home in Godric's Hollow, since the home his parents were living in was destroyed by Lord Voldemort. Perhaps he has a ancestral wizarding family home much like the Blacks and Malfoys, which has been lying empty for over 16 years, magically hidden by his relatives. I loved my grandmother's old stone house (still in the family--now my cousin's), and imagine the Potter house to be of a similar sort, only much, much older.