The MyTouch line are all vanilla Android so the upgrade is pretty simple. If the hardware will support it then it can be installed...more or less.
The Hero, Eris, Sprint Hero, Evo and Incredible all have Sense which makes things a little more difficult. Just like they did with 2.1, HTC will have to modify Sense to function with 2.2.
The good news of all this, HTC has said that they 2010 phones will get 2.2. This means that they will be developing 2.2 with Sense for the Evo and Incredible. This means that there will be a version of Sense out there with 2.2. The fact that the Mytouch line will be getting 2.2 also means that the Eris/Hero will support it. Maybe not the full version, but to some degree it can happen. So with the hardware capable and a version of 2.2 with Sense coming out anyway there shouldn't be too many reasons that HTC won't offer it for the our phones. The only thing we may have to worry about is Verizon. If Verizon decides that they don't want to offer an update then we may end up just being SOL.