here's to hoping it's an awesome update with lots of goodies and especially the ability to run multiple applications simultaneously (on the 3g and not just 3gs).
I think Apple will only announce OS 4.0 and it will release it in summer along with new iPhone hardware. Pretty much what they did 2 years ago when Apple first released SDK. Now OS 4.0 will be announced tomorrow because Apple Tabblet will run OS 4.0 as well.
I think that Apple will release the iPhone OS 4.0 by tomorrow because the iphone dev centre is closed and says come back soon.
I supose it shall be introduced at tonights special event.
They may release it to developers but I highly doubt the public. Developers need some time to work the bugs and beta test. Also, if it is a significant improvement in UI and such then it may be like the first SDK release and last in beta until summer because that would require a lot more testing.