Slowly but surely, Apple wants its early adopters to switch to newer models and it is making sure this happens by reducing the compatibility of the iOS 4.3 beta to the iPad, the iPhone 4 & 3GS, the iPod Touch 3G & 4G.
Those still using the iPod Touch 1G & 2G or the iPhone 2G & 3G are out of luck. That said, older models had less powerful processors and less memory which might make their user experience painfully slow.
iOS 4.3 beta, which was launched yesterday do developers and carries the build 8F5148b, is likely to be the one that will run on the iPad 2 when it launches in a few weeks.
The current trend, as it stands, is that Apple stop supporting portable devices that are older than three years or so which means that the flagship iPhone 4 may be EOLed as early as 2013.