The SDK is build 10M2305a and the firmware is build 8B5091b.
Developers can let us know whats new in the comments and we'll update the post. In the meanwhile here is the changes Apple made for the first beta of iOS 4.1.
Changes in Beta 1:
● Signal Bars fixed and taller
● New Baseband (02.07.01)
● Game Center is back with new look (see screenshot)
● Camera Switch and Flash buttons reposition in landscape mode (see screenshot)
● You can now choose to add Favorite as voice or FaceTime.
● Spell Check can be turned off in Settings
● FaceTime and Game Center Parental Controls
● Full Bluetooth AVRCP Support