To answer your question, I just undid the update to the old Froyo version of the market on my old Optimus V (which isn't active as a phone, it's just around for testing apps and whatnot). It's back to (what I assume is), the Donut version that the phone shipped with.
However, after I locked the phone and reopened the app, it is back to 2.2 Froyo version. I'll use the APK to update to the newer ICS version (which I will call the new one because it has the Honeycomb icon which will probably be used in ICS) and I'll see what happens.
Edit: Ok, I removed the update and it went back to Donut version (Eclair version?) and then I locked the phone for about a minute, went back, and now it's the Froyo version. Looks like the update will be removable HOWEVER it also doesn't look like the update is actually completely out.
So, with that being said, since the app has turned to the Froyo version from Donut twice now, I'm just going to say that once Google starts pushing the app OTA, you won't be able to remove it that easily and it will always revert back to ICS version from the one your phone shipped with (but this may be different if you're rooted).