Where I am, the situation is similar to what polly described where the Deans are all TT and have a PhD.
Dean of Students at a primarily undergraduate college is a big job, and I don't think it could be handled by someone that is maintaining an active research program and teaching.
I've not seen a Dean of Students who maintains an active research program, but I've seen them teach a class every semester because they love to teach. They are drawn from the tenured ranks, but I agree that it's a big job (even at the small schools where I have been) so that it's a different daily life, unlike being department chair where the tasks just go on top of being a normal faculty member.
It sounds like it varies between institutions. For clarification I didn't say the current Dean of Students had an active research program...though a few other Deans, and the president of the university, do. Our current Dean of Students is like polly described and as far as I know he 'just' teaches in addition to his duties as Dean.