As expected, Apple opened the download gate for iOS 4 today. The mobile operating system upgrade appeared to download and install with few problems, although some users complained that backups of their iPhones were taking much longer than usual.

Users on Apple's iPhone support forum reported total upgrade times, including downloading the update, backing up their iPhone and restoring it with the new operating system, that ranged from 20 minutes to over an hour. The iOS 4 update weighed in at almost 580MB.

However, some iPhone owners encountered problems in the backup portion of the process. "Stuck in 'back up' mode for 20 minutes so far-no visible progress," said "sclntx" on the Apple support forum. "Arrrgggghhhh."

"It looks as though it is sitting at the backup part, even though I synced and backed up before hitting the update button," said "scsutke92" on a thread dedicated to the problem.

"The Backup is 'glacial' slow but it is doing it," assured another user, labeled as "Nik Green," in a second support forum thread.

Other users reported different difficulties with the upgrade.

"I downloaded iOS 4.0 and it didn't install, then it went into recovery mode," said a user identified as "sohailkhan1993" Monday. "Every time I connect it to iTunes it doesn't come out of recovery mode, [so] I click on Restore and it attempts this but a message comes up 'The iPhone could not be restored. An unknown error occurred (21).'"

An Apple support document dated May 2 spells out error messages users may see when they try to update and restore their iPhones, iPod Touches and iPads.

The iOS 4 upgrade can be installed on 2008's iPhone 3G and second-generation iPod Touch, and 2009's 3GS and third-generation iPod Touch. iOS 4 is not available for the first-generation iPhone or iPod Touch, both which debuted in 2007.

Previously, Apple has charged iPod Touch owners $9.95 for major operating system upgrades, but an accounting change last year now allows it to provide iOS 4 free for the iPod Touch.

iOS 4, which Apple previewed in April, boasts more than 100 new features, including multitasking, folders and enhanced enterprise tools. Users can download iOS 4 by connecting their iPhone or iPod Touch to their PC or Mac, running iTunes, clicking on the device in the listing on the left and then clicking the "Check for Update" button.

iOS 4 is set to ship for the iPad this fall, according to previous statements by Apple. The company has not disclosed a specific launch date for the tablet's upgrade, however.

Although iOS 4 undoubtedly includes several security patches, Apple had not released an advisory detailing the number and severity of the vulnerabilities it addressed by 3 p.m. ET. Assuming Apple publishes an advisory later today, it would appear here, on the company's support site.