That's right. Every public tweet, ever, since Twitter's inception in March 2006, will be archived digitally at the Library of Congress.
....So if you think the Library of Congress is "just books," think of this: The Library has been collecting materials from the web since it began harvesting congressional and presidential campaign websites in 2000. Today we hold more than 167 terabytes of web-based information, including legal blogs, websites of candidates for national office, and websites of Members of Congress." (more at http://blogs.loc.gov/loc/2010/04/how-tweet-it-is-library-acquires-entire-twitter-archive/)
Seems like at the very least this could be used as an interesting anecdote to try to talk about how Libraries are changing. But seems like there could bigger teaching and learning. At the Academic Technology Showcase faculty from HIST 3611 talked about using twitter with prompts during a film and how much more engaged students were in the content. It would be interesting to see what sort of academic tweets are going on.