Also - your "multiple UI version" argues for separate databases if the version change requires a change to the DDL.
I also don't like being on WHERE clause away from a disaster.
Totally agree - keep the data as separate as you can. Run separate instances; I'd even go so far as to say - run separate virtual servers (a.k.a VMWARE). That way - when one customer starts dragging performance down for everyone, it's easy to move their instance elsewhere without SQL or App reconfiguration.
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Your lack of planning does not constitute an emergency on my part...unless you're my manager...or a director and above...or a really loud-spoken end-user..All right - what was my emergency again?