As someone who's gone a little too far down the template metaprogramming path in the past, I feel I should remind you that one of the goals of a UI library is that applications written with it should compile in less than 5 minutes. :-)

I don't see compile times being an issue with what I'm imagining. We'll see how it turns out, but most of the syntax I'm imagining will be a matter of clever operator overloading--template metaprogramming will be a factor, but not enough of one to really bog down compile times. In other words, it will be an event-based quasi-IM-style UI with a purely declarative syntax (i.e. UI structure is fully specified in a function which is called only once during program execution).

Like I said, it'll be an amusing prototype, but not something I'm currently expecting to want to use in a production environment. More an interesting programming exercise.