Its almost 2 days since the iPhone OS 3.0 was release for developers .iPhone OS 3.0 beta needs a developer ID for activation .Many non developer users tried to update their phone to firmware 3.0 form firmware 2.2.1 and now they are getting errors or reduced phone functionalities .By luck ,unlike in other firmware versions ,here you can downgrade
iPhone back to firmware 2.2.1 and restore all phone functionalities .Baseband of iPhone OS 3.0( 04.20.01) remains the same when we downgrade , but there no problem as far I have seen.Here is how you can change  iPhone OS 3.0 to 2.2.1.

Connect your iPhone to your Mac or Windows PC .Open up iTunes 8 and select your iPhone 3G from the device list .Sometimes your phone  will not be listed in iTunes ,then select device firmware upgrade mode .

How to put your iPhone in device firmware upgrade mode.

Switch off your iPhone 3G .
Press home and power button together for a suitable time , say 15 seconds .
Keep pressing home button until your computer/MAC detects new USB device .
Now iTunes will detect your iPhone connected in DFU mode.

Change  Apple iPhone OS 3.0 to 2.2.1

Now browse the summary tab .
“
If you are Apple user , press on option and click restore .
If you are Windows user press shift and click restore .
”
Now you would be able to browse for earlier firmware versions .Click on firmware version 2.2.1 and restore . Wait for 5-10 minutes before your iPhone OS 3.0 is changed to 2.2.1.

Sometimes you would get error messages 1011, 1013, 1015,1016 etc . You can just ignore them , your iPhone firmware 3.0 had already  changed.

If needed , use Quickpwn 2.2.5 to jailbreak/unlock your iPhone back to version 2.2.1 .
You have to repeat this procedure if  you are getting error massages 1601, 1602, 1600 or any related one .

Finally Apple has released the features of the new firmware update /OS version 3 for iPhone 3G and 2G.

There is something to be happy . As promised , iPhone OS  3 will contain enhancements like MMS ,A2DP ,push functionality , copy-paste etc. So finally Apple iPhone is about to set it as a complete multimedia device .You can read complete Apple iPhone OS3.0 features here .

Apple iPhone OS3.0 features

OS 3.0 will add MMS feature in all Apple iPhones.

Apple iPhone OS3.0 features A2DP (Stereo audio over bluetooth devices ) .

Peer to Peer connectivity over Bluetooth for file transfer and gaming .

Apple iPhone OS/firmware 3.0 includes much awaited copy -paste functions .

iPhone OS 3.0 adds much features for developers ,like access for external hardwares , use of streaming video and audio in there iPhone applications .

iPhone application store will be available in more languages (Approximately 77 ).

Parental control feature for streaming TV shows in iPhone .

Easy to use spotlite homescreen .It will give quick access to all important iPhone data like mail , contact media etc.

iPhone OS3.0 features new Apple applications and enhancements :

Phishing protection .
New application called voice memo will be available with iPhone OS 3.0. With this you can add and record notes ,remainders etc .
iPhone OS 3.0 adds auto fill feature for internet forms .
All apple applications were integrated to search functionality.
Enhanced Youtube account support .
Landscape view for mails and text notes .

Apple iPhone 3.0 /Apple iPhone firmware update 3.0 will be available as a free download for iPhone 3G/2G users ,by the end of march or first weeks of April .iPhone 2G users can also use OS 3.0 . But they won’t be able to get A2DP and MMS features due to hardware limitations .

iPod touch users can unlock the Bluetooth feature thorough iPhone OS3.0 ,buy paying $9.95 .

A developer beta of iPhone OS/firmware 3 can be download here .You can update your iPhone via PC /MAC and iTunes .I will provide the direct link to the final installer , after the official release .

Download Apple iPhone firmware/OS 3 developer beta.

Update: Apple announced iPhone OS 4.0. Checkout the new features of Apple iPhone OS 4.0 and download links.