I found a very good article on the iPhone OS Upgrade cycle.

This graph is really interesting:

iPhone OS

It looks like it only takes 1-2 weeks to get to 50% adoption, 1-1.5 months to get to 66% adoption, and 2-3 months to get to 90%.  And that is for upgrades that are less interesting than the 3.0 upgrade.  So, let’s say 1 month from next Wednesday’s 3.0 release, or mid-July,  for 66% and 2 months, or mid-August, for 90% adoption.