For a long time now, Microsoft has been arguing that service packs are less significant than they used to be. What was once taken as a sign that a product had the kinks ironed out is now more a psychological milestone than a technical one.
That said, many are still pointing to the release of Windows Vista Service Pack 1 as a key sign to businesses that the product is ready to be taken seriously.
Microsoft finalized the code for Windows Vista Service Pack 1 at the beginning of February and eventually allowed some technical users to start downloading the code. Broad release though, has been slated for "mid-March," which by my calendar means it should come this week. Mary Jo Foley notes that Amazon appears to be expecting the retail version to be available this week as well.