Likely close to when Windows 7 comes out.

FP

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I think we are in rats' alley
Where the dead men lost their bones.

An "SP" update for a Windows OS is a Service Pack. A service pack applies all previous incremental critical and optional updates to the OS, as well as a few larger fixes. One example of a larger fix: With Vista SP1, they updates the behavior for how Vista reconnects to wireless networks and manages those connections

Vista SP2 will likely be a year or more from SP1's release, maybe longer. It wont be until they've feel that the accumulation of SP1 + later updates should be rolled into a big Service Pack again.