>So far here is the score:
>
>Undecided: 1 (me)
>Data tier: 2
>Biz tier: 1
>
>Each of the respondents has had very good arguments supporting their position.
>

Where the RI goes is an implementation issue and as with most implementation issues, there is no clear cut answer. Enforcing the RI, by design, is a biz rule. Where it gets implemented depends on what your needs are. Most of the time it will get implemented in your data store. If you go with XML, you need to move someplace else.