At WWDC, it came out that half of all iPhone owners have never plugged their device back in to their computers after initial set up. In fact, after I got my iPhone, my entire family fallowed suit, and none of them (literally 0 out of 5 people) sync their device, update it, anything. It just not something that they know to care about. Too much of a nuisance, not enough gain.

Wireless updates (announced for iOS 5) should definitely help though.