This has been discussed a few times in the past, see these questions (plus the comments):
As stated in those questions, the idea of separating admin content so that each admin can only affect "their" store doesn't actually work the way you'd think. For instance, what if one store owner wants to add a new attribute to products? Or subtract an attribute set? Or add new categories, change configuration data, etc etc. Magento was not built with this in mind, and I cannot imagine being able to do this in a way that is airtight (so that admins cannot affect each other). Use multiple installs for this.
Hope that helps!
Thanks,
Joe
EDIT:
As a sidenote, this being the fourth time this question has been posted to SO, obviously there's a need here and a chance for someone to create a killer extension that others pay for. Someone take this one up and post it on Magento connect for all these poor folk. :)