The exact same thing's been happening to me lately, and it wasn't a problem before.
I was speaking tjol with some mates on IM and went to go get something to eat. Got back about 10 mins later and there it was: nothing! I could open
http://www.cellc.co.za but nothing else; my modem was still connected, with a soothing cyan-coloured LED and everything; checked MDMA, which said "HSPA+, -69dBm, tower 22492"; couldn't ping DNS servers but could still resolve stuff; and traceroutes to
www.cellc.co.za and
www.google.co.za look similar to what ginggs (I think it was) had posted earlier in this thread.
Win7 64-bit, Centurion. I've already disabled all the idle timeout / power savings settings I can think of since it does sort of feel like an idle timeout issue, but I never had this problem during my first 2 months of being with Cell C (Dec. 2010 and Jan. 2011) and back then all those settings were still enabled, so to be honest... I don't think it's me or my PC or my bad karma or anything like that.
Please, Cell C. I cancelled my Telscum ADSL in order to get a connection that's both WHOOOOOSH and (more importantly, to me) S T A B L E, so it would be stupendously satisfying to me if you could perhaps look into this and maybe even fix it. Thanks!