One of the things I was looking forward to in my next company was not thinking about the battle between Flash, Ajax/DHTML, and more raw/native rendering technologies. It turns out I can't escape it :-). I'm now working for Me.dium (cool idea, cool people) and we're facing the same battle. Ajaxian just did a great writeup on what Me.dium's up to. We're blurring the line between client-side app and web-based and have currently built our product using Ajax. We just fixed a bug that drastically improves our performance (so update your extension to get the latest), but the debate rages on as to whether or not we should stick with Ajax or move to something else.
We need the usual... cross-platform and cross-browser support. We need highly performant apps, and we need to consider ourselves a web-based app so we're not updating client-side bits every time we want to make a change.
We also need the ability to zoom our UI which implies vector based graphics (e.g. not Ajax :-( ). This requirement could push us into Flash in a hurry. We could use more native/raw rendering, such as canvas, to do what we need, but then we'd have to build an entire framework (layout, data-models) ourselves which is more work than anyone would prefer to get into.
I wonder if I'll ever be able to escape the madness of rendering technologies.