I can assure you that it was *not* closed off (at least as late as 9:30, when my better half and I were done with dinner), although there were scads of folks in costume. Traffic was slow, and cops were directing traffic, but I did not even see any streets closed off.
There were crowds near the remaining bookstores in Harvard Sq (the Coop/Barnes and Noble, Harvard Book Store, and Curious George Goes to Wordsworth), and one could buy a ticket to see a HP movie at the Brattle Theatre that came with a copy of the book.
--tim francis-wright