IE7 is coming out very soon (some say any day). One thing to be aware of: if you need to keep using IE6, don’t install IE7. This isn’t just an issue in the IE7 installer (i.e., IE7 upgrades IE6), but they cannot both be installed on the same machine.
I trust (and Microsoft really hopes) that this doesn’t affect users in a negative way.
My guess is that the only people who really care about the side-by-side issue are developers. Of course, IT managers will care too, but they can keep IE7 from being auto-updated.
My guess is that this due to COM (because IE programmability is all through COM).
Side-by-side COM is anywhere from hard to impossible. One more benefit for a browser built on managed.