I’ve channeled all my google-fu into finding an answer, or leads on an answer. The first clue is this page which is trying to find the first Twitter user. It seems user numbers 1–11 are not currently active, so I’ll assume they were used for testing, and thus the first ever post made on the service we now know as Twitter was likely done using those testing accounts, which would have been deleted with the now-inactive 1–11 user accounts. (That’s how I test my web applications – get everything running smoothly, do some test posts, wipe the database and use that same wiped database for the live site, so the primary keys may still be saved in memory and thus the first row added in gets a primary key of 12 in the case for the user accounts).
That page also shows the same post which is seen in both of your links about setting up twittr (note the spelling). Because all three messages are identical, and because the timestamps on your two links are minutes apart, I conclude it is an automated post made by the server when the user account was created for the first time.
So my final conclusion is that the first Twitter post is likely a tie between the ones you’ve posted and any similar one that may be in the twitter account “jack” which is the oldest account on Twitter and logically the founder’s account. Very likely, the true first post on Twitter has been deleted during the testing phase.
OH! Check it! I’ve found it! http://twitter.com/jack/status/20 is, as I predicted, the same message you found, but dated one minute in the past.
So if you count that post as the server posting, the first true Twitter post is likely this one from Jack, as the only other accounts in existence at that time were (presumably, but not with any proof here) biz and ev, and neither of those accounts have a post at the URL between 20 and 29.
What a mystery! That was fun to unravel :)