iOS 4.3 will be released to the general public next Friday, on March 11. Today, Apple uploaded the gold master seed for developers. The release remains under the NDA umbrella until next Friday, however, with details outside of those publicly discussed by Apple remaining off limits.
The new 4.3 firmware will include many new improvements, including enhanced AirPlay and Personal Hotspot support.
If you are a paid developer, head on over to the Apple iOS developer site to download your SDK and firmware. You will need to sign in with your credentials in order to get behind the paid firewall for these items.