Leave it to Amazon to clear up Microsoft's obfuscations.
A search on the term "Windows SP1" returns several versions of Windows Vista, including Ultimate and Home Premium editions. The suggested manufacturers' price for Vista Ultimate with SP1 is $319.95 ($299.99 on Amazon) and Vista Home Premium with SP1 is $239.95 ($215.99 on Amazon).
On Monday, my colleague Ina Fried tried to nail down the exact shipping date from Microsoft. She did find out that Windows Vista SP1 is expected to be available in retail stores "as soon as April," according to a company spokesperson.
The main features expected in SP1 are fixes to avoid incompatibilities with some applications as well as better reliability, security, and performance.
Mary Jo Foley at ZDNet also spotted the Amazon listing (with a hat tip to Computerworld) and reported that Windows XP SP3 is not far behind.
A report from the Associated Press, meanwhile, suggested that Vista SP1 could be available for download Tuesday.
Update 5:21 a.m. Pacific: Microsoft finalized the code for Windows Vista SP1 in February and made the software available for download to members of its MSDN and TechNet developer network in late February.