Bootstrap 3.0 is currently looking like it's going the flat route, though it's still rounded. You can check it out on the 3.0.0-wip branch on Github.

there is also http://web2dev.me/metrostrap/index.html and as someone else pointed out bootstrap 3 is more or less flattened out as well.

I guess its a trend thing, when it became to easy for regular developers to do designers job that used to take hours in photoshop with css3, gradients wasnt that interesting anymore...

Flat Strap is good but it lacks with components

I'm not affiliated with the project but please do open an issue over at Github.