The SDK 3.1 and 3.1 OS beta versions are released for iPhone by Apple, but right now its limited just for developers and it has improvements from the previous version. i.e. the iPhone 3.0 OS.
But if you are not a developer and still run to dev center to download and use it, be WARNED that you won’t be able to unlock your phone if you are upgrading.

Here are a few features that one would see in the iPhone OS 3.1 update -

• Added the new option “Save and Copy” while editing the video. This option is good to keep the original video aside and play with the cut-down video, and earlier you were able to just cut the video and save it directly replacing the original video.
• Better reception of network
• Improved WiFi Reception
• Voice control working with bluetooth
• Multiple SMS sending possible now

The features surely got better, but the warning still remains the same for everyone -
iPhone OS 3.1 would come with upgrade to your baseband to 5.08.01 so that you won’t be able to go down to previous OS or unlock your iPhone.

So, if you are using an unlocked iPhone 3G, which was done with ultrasnow then be sure to be away from this OS release. This update would be useful for only those users who don’t care about iPhone unlock and run with contracts with the networks.

And the iPhone 3GS users will have to wait for any unlocking help from iPhone Dev Team as they are delaying it due to lesser number of iPhone 3GS users needing the unlock solution. It would come out when the demand increases