We had a problem of disabling DatePicker field in javascript. Although the input can be disabled, the calendar link can not. That gives some weird feeling. We finally make it disabled with JavaScript, but it may not be future-proof. It may not work if Tapestry changes how the DatePicker renders.
I think that Tapestry do not need to be a fully ajax capable framework, but I think Tapestry should be ajax friendly. I mean that Tapestry pages should be easily enhanced by javascript. Just my two cents.
