In XP I run a customized MMC with all my AD, DNS etc. I also run the Exchange 2003 System Manager snap-in from there. In Windows 7 I have been unable to figure out how to add the ESM (or any) additional snap-ins. It used to be that if it wasn't in the list there was an Add button. Now it seems that if it's not in the list you're done.
If I could find where all the "available" .msc files are kept I could put the ESM.msc file there but I can't seem to find that either.