Summer of this year..the reason you need to be a developer to get access to the 4.0 OS is because if you do something stupid, you know how to go back to an older version. Also, many developers usually have multiple devices they use for development. The iPhone OS 4.0 isn't meant to be used on your main device as of yet..
So before you risk turning your phone into a beautiful paperweight..just hang for a bit..and in about 2-3 months we'll all have access to 4.0..
What I meant is that you will be up under someone dev account. If Apple revoked their account, your phone would be useless. Also, you have to update the firmware when each version comes out wether you like it or not as the old one will expire and brick the phone. The point is that I would not bother with the update until Apple releases it. I am a dev and I have yet to update my primary device as it isn't worth the risk at this point.
My jailbroken comment was only that you can turn off the self destruct that Apple has built into their code. I do not know if this will be possible yet with OS4 as I have yet to use a OS4 jailbreak. If the features I want are there when it is released, I will likely not even bother with a jailbreak.
I'm not clear on why you would want to use a bug-filled and unstable version of the iPhone OS at this point anyway. iPhone OS 4 isn't done -- it has issues. That's why its being tested.
I don't know about some of you, but I use my iPhone to do serious stuff (not the least of which is make and receive calls). I have zero interest in putting OS 4 on my iPhone before its been released, since I would have zero recourse if something went wrong.