I've written AJAX apps and even an AJAX theme for WordPress (not that it was great or anything–it wasn't) and I decided Widgets would be better off without it. So Widgets uses good old HTML forms.
That doesn't mean some brilliant widget developer can't put a slick AJAX interface into a widget or its configuration module.
And finally, where did anybody get the idea WordPress 2.x will drop support for 1.5 themes? It won't. We're expanding the theme system and making it more powerful but we're maintaining compatibility with WordPress 1.5 themes.
That doesn't mean all 1.5 themes will always work. Some advanced themes "break the rules" by accessing the database directly or otherwise bypassing API. Those themes will almost certainly break. That's the price you pay for developing a theme beyond the capabilities of the API and not submitting a request for API improvements. (the bugtracker)