Great stuff!
var tag = React.createElement
Could make the code nicer to read... var tag = React.createElement
Could make the code nicer to read...Personally, I moved from using DOM tag names as variable names ('a' being the worst) to saving 'var DOM = React.DOM' (or '{DOM} = React') and then calling DOM.a(props, children...)
Check it out: http://mithril.js.org