I have to immediately thank Dave Delaney (Twitter: @davedelaney) for this tip.  Revealed in his 5,000 tweets and counting post earlier today, he suggests the following to find your first tweet:

1. Browse to your Twitter profile page
2. Click on the “Archive” tab
3. Scroll to the bottom
4. Click on the “Older” button
5. Notice that the URL is now something like http://twitter.com/rickmahn?page=2?  Change the “page=2” to “page=200”
6. See anything?  Start decreasing that number until you find your first post.
7. Or, if you’ve been especially productive, you may need to try higher page numbers! 😉

Mine just happens to be on March 4th, 2007 at 1:13pm (found here) where I tweeted the very informative “Reading RSS feeds” to the world.

So, thank you Dave for that tip, and congratulations on crossing 5,000 tweets.  I had been curious about that first tweet lately because I know I was starting to get close to a full year on Twitter.  Just over a month to go!