What are the top-tier programs for an MFA in creative writing,
concentrating in poetry, in the United States?

You might want to give the University of Illinois a look. They have an
excellent MFA program, with a roster of very impressive names as
lecturers and readers. For example, the great poet W.S. Merwin has
participated in classes there. Don't know about you, but, as a poet, I
would walk ten miles on broken glass to be able to discuss poetry with
W.S. Merwin!

http://www.english.uiuc.edu/mfa/content/program.shtml

Thanks! I actually went to UIUC for my undergrad, and the thought of
spending another couple years in Urbana...well, I guess it's not that
bad, but I'd like to go someplace different.