[enathu]

This can be easily achieve by using the Twitter's newly added feature Tweet Button!

Here is how you can make your longest tweet.

Just type this URI on your address bar http://twitter.com/share?text=&url=yourtext

Change yourtext to whatever you want (make it more lengthy and use whatever language you want) and bang! The longest tweet created! It's working simply because of the validation error in the url shortening of t.co, I reckon! Hope this might be fixed very soon. But, until then you can make longest tweets after all! Happy tweeting.. oh. happy looongest tweeting!!

[jamesbritt]

Well, not quite ...

    Request-URI Too Large

    The requested URL's length exceeds the capacity limit for     
    this server.

Maybe 50+ paragraphs of lorem ipsum was a bit too much.

EDIT: 100K chars didn't quite make it, but ~1K seems ok

[enathu]

Actually you can make several t.co links and merge it together to post it on a tweet. Actually the tweet that this thread linked to have 5 links itself. Simply merging many t.co links with text can make the longest tweet and it works! And 50+ paragraphs is too much for a single URI anyway.

EDIT: Seems now Twitter has fixed the bug and only shows the t.co link not the text or exact URL in the tweet! No more longest tweets! Only t.co links.