Yep, almost a certainty. And per the interview with Google posted pages back here, Google is ok with that.
I'm looking forward to ICS with Sense on my 3vo - for me, that's just more choice.
The vanilla Android phones of the past did not sell well. If they had, then that's what the makers would likely embrace to a greater degree.
Right now, it's about differentiation and choice - and I'm all for choice.
I think the ICS/Sense integration is going to be very interesting. Sense is more than a launcher, it's an app integration scheme, much like a part of ICS is - so putting those two together without collisions should prove interesting. (Although - if drop Sense 3 for Sense 3.5 (aka Sense Zero) on my phone, I may or not accept the upgrade.)
In any case - people who care most about the stock Android experience that don't / couldn't buy a vanilla phone tend to root and get some AOSP rom, don't they?