Or is this the exception to the norm? I know that it's allowed at Brown - but I wonder if it's unique.
At many (most? all?) universities, a committee is required to have at least one professor from outside the department (other than the chair, if the chair holds a joint position).
Other requirements are to have at least one professor at rank higher or equal to the rank of the chair (the advisor).
As far as I know, there is no international guideline on what is a correct PhD committee, so it might be hard to answer your question, because each university might have different rules. For instance, the rule of the EDITE (the CS grad school shared by several universities in Paris) for defending a PhD at Paris 6 are (source, in french):
- the reviewers of the PhD (i.e. those who read and approve or not the manuscript, which is the most crucial part of the process) must have an habilitation (i.e. they could apply to full professor positions, even if they don't have one) and must be related to a different grad school and university.
- the PhD jury (or defense committee) should:
- contain between 3 and 8 members (the reviewers might not be included, although they usually are)
- contain at least one professor from Paris 6, who must be different from the PhD advisor
- contain at least half of people not related to the grad school or university.
- contain at least half of professors (or academics with an HDR).
So, to answer your question, in order to defend a PhD in CS at Paris 6, you need to have reviewers not coming from your university, and half of the members of your defense committee needs can't be from your university (although technically, they don't have to be full professor, so you can have Associate professors from another university, or even someone working in a company).
EDIT: After reading Jeff's comment, I realized that I understood "department" as in "university", and not as in "physics department" if you're doing a PhD in the "CS department". Then, in this case, there is no rule at the EDITE, in favor or against, but I have never seen it or heard of it, apart in the case of an interdisciplinary PhD (e.g. bioinformatics).