If users decide to manually access SP1 now, there's a chance they won't see it in Windows Update. There could be several reasons for that, according to the Windows Vista blog. For example, users of an earlier version of SP1 must first uninstall the older version before reinstalling the final SP1.
In addition, Windows Update will detect if a user has certain drivers that must first be updated before installing SP1. Until those users have the updated drivers in place, Windows Update won't offer SP1.
Initially, SP1 is available in English, French, Spanish, German and Japanese. If a PC has any other language installed, SP1 won't be offered to the user. Microsoft plans to release the second wave of SP1 in additional languages in April.
The company has long said that SP1 would come out in the first quarter. A major hint that the release was imminent came on Monday with an Amazon.com listing of Windows Vista that included SP1 and said that the product would be available on Wednesday.
It's been a long trek for Microsoft up to the release of SP1. Microsoft inadvertently posted SP1 to Windows Update in February, although few people were actually able to download it from there. Microsoft also had some hiccups with an update designed as a prerequisite for SP1 that caused some users to get stuck in a reboot cycle.
Some observers believe that the release of SP1 could boost Vista sales, particularly among enterprises that have said they're waiting for the update before upgrading to Vista.