It's really hard to say.
The iPhone 3GS hardware is similar in many ways to the iPad Version 1. The iPad does have a faster processor (although it's single core, like the 3GS). The RAM is the same, but the iPad has a faster graphics processor, so animations are smoother.
If the limitation on iOS 5 for the 3GS is CPU speed, or "animation" speed, then I think the iPad version 1 has a good chance at getting iOS 5.
If the limitation on iOS 5 for the 3GS is RAM, then I'm afraid the iPad probably will cap out with iOS 4, which would be a shame.
As others have pointed out, sometimes Apple will have partial support for an older product. For example, allowing iOS 5 to run on iPad version 1, but with some key features (voice input? 3d display? split screen iPhone Apps? 😀) disabled.
We'll probably find out in a few weeks at WWDC 2011.