None that I can find, but I find this to be very interesting question. A search-able index of tweets could be very helpful in my sales job. I sell specialty chemicals and if people are tweeting about certain things I could target those users. Probably not what Twitter inventors envisioned the use of Twitter for, but what the heck…

Google has a lot of this data. If you include site:twitter.com in your google search you’ll get just things that have appeared in Twitter.