Flash is not going to go away tomorrow. But it will be increasingly marginalized. A lot of people saying things like "HTML5 can't replace Flash because it doesn't work in Internet Explorer" are missing the point. The web is a pretty flexible platform. It's not a matter of switching from using only Flash to using only HTML5 video overnight.
What will happen instead is that sites will switch from using only Flash to using Flash, with an HTML5 fallback for mobile browsers. (There's already a lot of this for the iPhone.) As browser support grows, they'll eventually switch to using HTML5 with
Flash as the fallback, for older browsers. Eventually, when enough browsers have been updated, they'll just stop bothering with Flash.
At least for video, this will be very, very easy for web developers to do. Take a look at, for instance,
this HTML5 player. It currently supports Safari and Chrome. The released version is going to support Firefox via HTML5 as well, and also do automatic fallback to Flash for IE. There will be a lot of approaches like this, customizable in various ways, and once they're available embedding a video via HTML5 with Flash fallback will be a matter of one line of JavaScript. Once Firefox comes around on H.264 (which is inevitable, eventually), you won't even need to encode more than one version of the video file.
As far as the DRM thing... well, some content providers should really just get over it. No other media (text images, etc.) on the web is DRMed, why should video be different? Content providers who believe their content is so high-value that this is unacceptable can just write native apps for the non-desktop platforms they want to support. Native apps will work much better on most embedded devices than browser-based Flash approaches ever would anyway.
