Re: memories of 9/11, they have faded for the college/university students of today. I teach courses/modules that overlap the study of terrorism. Years ago I asked students what they remembered of 9/11; more recently the responses have been to the effect that they were in school and they only heard about it after they got home (or teachers pulled in tvs to show in classroom, but that was more frequent for high school than middle or elementary school).
To repeat, it was mostly college kids doing the celebrating, at least initially. The students weren't just frat boys; nor were they drunk (perhaps with a couple of exceptions). I teach at a university here, but have connections (faculty and student) in the States. I had former students celebrate at the White House. And current faculty in the DC area posted re: their students' activities. The announcement took place on a Sunday night while students were studying for finals that began on Monday morning at American University and GW in DC. And finals begin at Georgetown later this week.
This is a very wired generation; they keep in touch with each other constantly by text and other means. Once they realized what happened, they began to congregate and celebrate. If you look for it, you will see this happened at college campuses across the country; look for videos of Penn State, Ohio State, Iowa State, Boston College, Dickinson, U/Delaware, Appalachian State, UMd/College Park. Probably many more.
Those same kind of events also took place in Washington and New York, with the local area university students heading to the White House and Ground Zero (respectively). There have been multiple articles published about the college student celebratory response to the announcement of UBL's death, including in the Washington Post, University websites (sample here:
http://auambassadors.wordpress.com/2011/05/03/american-university-students-celebrate/ [nofollow] ), Slate (which described the crowd outside the White House as mostly college/university students), etc.
There will probably be more reflections in the days and weeks ahead about why they celebrated that way. They are trying to understand it themselves; see:
http://www.washingtonpost.com/blogs/compost/post/why-we-millennials-celebrated-when-osama-died/2011/03/03/AFx9TMcF_blog.html [nofollow]