Well, MT has gone to version 3.0 and changed their licencing terms and conditions as well as making the product more expensive. The backlash has already started but no doubt people will migrate away from MT, and when they do, Free Software like b2++ and WP will be waiting for them!
If it could be made easy for them to pull all their content out of MT and dump it into b2++ or WP you’d see faster adoption rates. Data migration is a big issue for MT users who don’t want to pay for MT.