Just-announced Verizon iPhone 4 runs on iOS 4.2.5 8E128. AT&T iPhone has iOS 4.2.1 8C148. Difference in OS could be to allow the use of Personal Hotspots in Verizon iPhone.
Shipping February 10, Verizon iPhone will run on iOS 4.2.5. It’s still not clear whether AT&T iPhone will also get the iOS 4.2.5 update or not.