At noon EDT on 22 Oct (launch day), Win 7 market share stood at 1.40%
Current peak is 2.90%, hit at 25 Oct 4:00 EDT
Daily peak usage occur at roughly 4:00 EDT
It'll be interesting to keep an eye on the trend over the next few days.
[UPDATE: This data is based on live market share as measured by Net Applications. The company measures OS and browser usage by tracking the computers that visit the 40,000 sites it monitors for clients, which results in a data pool of about 160 million unique visitors per month. So the market share fluctuates based on web usage.]