[fizx]

Twitter will likely crawl the final targets of the urls, so that when bit.ly goes out of business, the t.co urls pointing to bit.ly can get redirected to the canonical link. So I don't think it's quite as bad as you're thinking.

[DougBTX]

Apparently if you have a unique word in the expanded version of your url, then search for your tweet containing only the shortened url using the unique word, you will still find the tweet. I've not tested it myself, but I took it as evidence that they are indeed caching fully resolved urls.

[paulgb]

This is true. For example, few of these tweets contain the search term: http://search.twitter.com/search?q=news.ycombinator

Hopefully they'll be resolving all the way down the redirect chain.