The iPhone had been a gold mine for AT&T, the exclusive carrier of the trendy smartphone in the U.S. But the success is also proving to be something of a curse. AT&T is finding that it’s network is strained by the demands of nine million, data-hungry iPhones. Download speeds on AT&T have slowed to a crawl, upsetting customers who use the iPhone like a portable computer. Some report that it’s almost not worth trying to access the speedy 3G network during peak times in some cities. Like rush-hour, it is simply too clogged with other users. AT&T says it plans to spend nearly $18 billion this year to upgrade its networks. It needs to move fast, not just to handle the iPhone surge, but to salvage its reputation.