We’ll begin rolling out this change today across our Android and iOS apps, on twitter.com, TweetDeck, and Twitter for Mac. It will continue to roll out worldwide over the next few weeks. If you can’t wait to try out longer Direct Messages, be sure you’re using the latest versions of our apps so you get the update right away. Sending and receiving Direct Messages via SMS will still be limited.
I tried to send a long DM from Twitter for iPhone and iPad, but I'm still stuck on the 140-character version there. However, sending a 600-character DM from Tweetbot for iPhone worked fine.
The strength of Twitter DMs is, for me, the existing graph between users (people I'm interested in), speed, and the lack of baggage from email. Lately, I've come to like the ability to easily share links and pictures in DMs as well. I don't know if raising the character limit to 10k characters will by itself improve DMs, but Twitter is wasting an opportunity with DMs, so maybe their new CEO could use this as a starting point.