Excellent. I was looking at CSSTransitionGroup a few days ago and would have definitely have used this if it was available. Baking in a couple of default transitions is a great idea. Are you planning on creating an npm package?
However, I'm not sure about how to let user provide his own transitions, especially in cases where they'd like to use this in multiple controls - would something like require('velocity-transition-group')({ myTransition: {...} }); work for you (so that we wouldn't be passing transitions to each instance)?
Plus, there is this issue I found: https://github.com/facebook/react/issues/2456 - it seems that if eg. you use require('react') while a component uses require('react/addons'), they both seem to get 2 different instances of React, and the events don't propagate from one to another. I'd like that to be resolved before packaging this into module people will use without reading the manual :).
Edit: Thinking over it, I also need to read about how does browserify + react + components as standalone modules work out: Can it happen, that if a component specifies 'react' dependency in package.json, and containing project uses different version of React, that browserify would actually package 2 versions of React into a resulting file? That would really really suck!