Codenames for all iOS versions:

1.0: Alpine (1.0.0 – 1.0.2: Heavenly)
1.1: Little Bear (1.1.1: Snowbird, 1.1.2: Oktoberfest)
2.0: Big Bear
2.1: Sugarbowl
2.2: Timberline
3.0: Kirkwood
3.1: Northstar
3.2: Wildcat (iPad only)
4.0: Apex
4.1: Baker
4.2: Jasper (4.2.5 – 4.2.10: Phoenix)
4.3: Durango
5.0: Telluride
5.1: Hoodoo

That's pretty cool. In case you haven't guessed, these names are mostly after ski resorts.