MM> Would a young man be rejected for a computer science type
MM> course at MIT, and apply to Harvard with a reasonable
MM> expectation of success? What about the other way round?
Much depends on why. Generally at Ivy League schools one simply
applies for the undergraduate degree, and selects the degree course
later -- so your young man rejected at Harvard would be rejected in
general, not just for the computer science degree.
Both Harvard and MIT are extremely selective, but they select for
different things; someone who doesn't have the technical education (or
the visible potential thereof) to be accepted at MIT might have other
qualifications that Harvard is looking for. In neither case is
admission entirely objective; it's possible for a borderline candidate
to be accepted or rejected based on things like family history with
the school, athletic accomplishment, or odd hobbies.
Practically speaking, as I understand it, Harvard and MIT allow
cross-enrollment, so you you can be a student at MIT and take classes
at Harvard, or vice versa.
Charlton