I think 3GS gets an update.
Why ?
iPad 1 = iPod Touch 4G or iPod Touch 3G(32 GB/64 GB) = iPhone 3GS = 256 MB RAM.
All have same performance and why would apple leave out iPad 1 Users ? I mean iPad is an expensive one ($499 Vs $199 iPod Touch) and i am sure they will support it.
A4 on iPad 1 is 25% faster than the one present on iPhone 4/iPod Touch.
So i definitely think considering RAM theory , i think they will upgrade older devices but wont give exclusive features.
Example :
1.iOS 5 has Integrated Siri in 4S and Dictation on iPad 3 but not in iPad and iPod Touches.
2.Location based reminders won't work in iOS 5 of iPod Touch and iPad.
3.Facetime over 3G not available for Wifi devices (iPad/iPod Touch and 3GS has no front camera but has 3G , so may be available but faces cant be seen directly face-to-face)
Conclusion :
All will be upgraded for sure but not all features supported by all.
Some Things which must be considered :
Apple apps like iWork , Garageband will be upgraded so that they need latest iOS , so who are using Apple apps on iPad 1 need to upgrade for sure (iWork was introdudec along with iPad 1) to get latest versions working.Currently all apple's app need iOS 5 as requirement.
But most 3rd party apps work like a charm.
Apps like Cut the Rope even work on iOS 3.0 (it is one of the only popular app which is updated constantly and supported by iOS 3.0 onwards).