As Kabaldan noted, there's no way to know for sure until the sources are released, and that should take a little while still.
My guess: No, I don't think ICS is going to happen for the Milestone, at least not in an useful way. Along the history of Android releases, no matter what Google said about how a new release compares against an older release in resource usage, every new Android release always required a little bit more RAM than the release before: Froyo required more RAM than Eclair, but as long as you don't stuff your phone full with apps and you stick to a relatively "light" usage pattern, the Milestone still handled it pretty well. Then Gingerbread again required more RAM than Froyo, but now with Gingerbread we're really on the edge of what Milestone can handle - if you don't stick with a light usage pattern, you are sure to run into apps you need open being closed because of low RAM. I just can't see the situation improving, or even at best staying the same, with ICS.
I think a phone with at least 512Mb (like, say, SGS1 and many others released in the past year and a half) is most likely to be able to handle ICS with little downsides, but the Milestone obviously isn't in that league. The Milestone is a 2+ year old phone anyway.