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Yes, but the iPad was then updated and sold with iOS 4.2 pre-installed.

iPad is the same hardware as the iPod touch 4G but with the A4 clocked faster. It should run iOS 5 just fine.

I think iOS 5 will see the end of support on all non-Apple-branded processor devices. So the 3G iPod touch and iPhone 3GS get the boot.
It is not always technical reasons that prevent an older iOS device from running a newer version of iOS. Apple can decide to allow the iPad1 to run iOS5 but not take advantage of all of the capabilities. Just like the 2G iPod Touch could run iOS 4 but didn't fully support the multitasking capabilities of iOS4 nor could you set a background image.
 
I hope Apple makes multiple versions of the OS optimized for the individual devices. The OS for the iPad 2 shouldn't be held back by the iPod Touch.

They are optimized by omission. The latest/most powerful devices receive the full feature set, while older hardware misses out on some functionality. For example the iPhone 3G didn't receive voice control or multitasking.

It's in Apples best interest to keep the same core iOS version on as many devices as possible. This ensures that the current APIs exist and help maintain App Store compatibility (Fragmentation is bad). Unfortunately there comes a point when even a stripped down version of the OS no longer runs acceptably.
 
Unless it is specifically tailored for the iPad 2 (the only IDevice with a5 chip) it should be fully supported. I'd expect any iOS compatible with iPhone 4 to also be compatible with first gen iPad since they have the same chip, and ios5 will obviously work with iPhone 4
 
Maybe iOS 5 will have a new UI that requires a higher resolution display than the 3Gs. If so, then all of the iDevices with an A4 or A5 would be supported.
 
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