1. searching tweets through twitter's search engines will only give you the last 6 days of tweets
2. tweets are now searchable on Google.com
3. the tweets on Google.com go back much further in time
So if you are searching on twitter, and can't find what you're looking for, you might want to try searching on Google, and see what you pick up.
In order to find my tweet(s) about the screenings of the new Phil Ochs documentary I did a google search "screenings raybeckerman" which immediately produced what I was looking for:
The search results page produced lots of relevant tweets:
Supposedly, if you want to restrict your Google search results to tweets, you can start off with the following parameter: site:twitter.com I.e., if you wanted to do a search for references to the #philochs hashtag on Twitter, your search might be: site:twitter.com #philochs & hashtag. But, based on my playing around with it, it seems that using this method may, for some reason, limit the number of tweets you unearth.
I don't know if this works with other search engines. I tried it on altavista.com, and it did NOT work.
In case you're wondering why you can find the tweets on Google but not on Twitter, I'm guessing the answer is M.O.N.E.Y. Probably Twitter has been holding back on its search results to make searchability of tweets valuable. And Google probably paid for the access.
And if you're wondering why they're not showing up on Alta Vista or Yahoo ... I guess those search engines didn't cough up the dough Google coughed up (or maybe Twitter gave Google the exclusive).