Yes, but remember that the iPad 1 was discontinued in early 2011 (1.5 years before iOS 6).
Apple's general rule of thumb is that as long as the device is still being sold, give ONE major update. That's pretty much how it's been treated*. I think the iPhone 1 had a couple updates, but that's the biggest notable exception.
I think it'll be interesting to see how Apple handles the iPad 2/3 thing. The iPad 2 is still being sold, yet the 3 isn't... will they support the 2 longer?? My gut says no. But it's certainly interesting.
*I don't know about iPod touches. Honestly I couldn't care less about those. But that's how it's been for iPhones and iPads.
The 3GS came with iPhone OS 3 and still runs the latest version of iOS 6. That's 3 major updates.
iOS 7 will run on iPhone 4, iPod Touch 4, ipad 2 and newer as these are still being sold.