I saw a major increase in download speed after loading iOS5 on my AT&T iPhone 4. Using Speedtest.net, I'd been averaging 9-10 Mbps download on the iPhone, compared with 19-21 Mbps download on a laptop (same location in house, same test server). After iOS5 I just got 18-21 Mbps in 5 tests on the iPhone, Not sure what Apple did, but I'm happy with it.