I pay everything, including the lines for my parents. Call it part of helping out for still living with 'em (you give me more money, I'll move out!).
I started off with my own Cingular phone plan about 5 years ago because I wanted to really have my own phone that wasn't prepaid. I was on Virgin Mobile, which was OK, but the phones were eh and the network was Sprint's "if you're near a big highway..." plan.
I got sick of my parents being on PAYG plans because they NEVER had their phones on. Got them on my plan, mainly due to defaulting by so-called friends of mine. It's $100/month for the three of us (1 iPhone, 2 other phones), which really isn't too bad. I have 5,000 rollover minutes and a unlimited minutes pretty much anytime that isn't peak time. I seriously can't use up my minutes quick enough.
I sure as hell don't think I'd ask my parents to pay my $65/month iPhone plan unless they were ridiculously rich. I just wish I used my voice minutes AND my data connection enough to justify it all. I use a fraction of what I am allotted. The day AT&T comes out with a cheaper data plan for less use, I'M ALL OVER IT.