I've been using the YouTube HTML5 video trial with Firefox for about a year now, because I am religiously unwilling to install Flash on any of my machines. From my experience so far, it seems like about 50 to 70 percent of the videos I click on are viewable without Flash. The weird thing I've noticed, though, is that some times, even in a series of ten videos on the same subject done by the same author, half of the videos will by HTML5 viewable and half will not. You would think that if an author uploaded a ten part series that they would all either be HTML5 compatible or they all would not.

My question: When someone creates a YouTube video, do they have to upload an HTML5 compatible video format (for the video to be viewable in the HTML5 trial)? Or does YouTube somehow convert whatever format is uploaded? In other words, why are only some videos HTML5 viewable, and how much of that is under the uploader's control? (As you might have guessed, I've never uploaded a YouTube video.)

The beta HTML5 player on youtube is exactly what it is; A player. The videos themselves are not to be converted, it's just that the player is written in HTML5. What browser and version are you using?

Firefox 8.0. Something must be converted, unless all the videos are being upload in WebM/V8:

(https://www.youtube.com/html5)

I know the Linux version of Firefox doesn't play h.264, only WebM/VP8. Don't know about the Windows version.

I was talking about on the user end before; As far as the actual formats though, have you tried using a different browser, like chrome?

Last time I checked (a couple months ago) there wasn't a checkbox on the YouTube upload page to make a HTML5 Video compatible version available. Whether a Webm version gets created seems random and Google are taking their own sweet time converting existing videos.

Which is why I think Chrome and Firefox dropping h264 support was a dumb idea as most web videos and all YouTube videos are already h264 as Flash uses that. Especially considering MPEG-LA dropped its demands for license money from Mozilla (and promised never to do so in future to any open source project) and the license wouldn't exactly hurt Google's balance sheet.

Also don't forget videos with ads don't get shown as HTML5 Video.

EDIT: Clarified

I'm not sure whether this belongs here or I ought to open my own thread but I have the opposite problem to the OP:
When watching a series of videos in YUoutube I will be presented, at random, with an HTML5 video instead of a flash player and even if I close Firefox and reopen it I'm still presented with the HTML5 version of the video once I've found it again. My workaround is to open the copied URL in Chromium which opens it in Flash.
As I understand it from some googling this may even be by design on Google's part as I've seen a post on one of their help forums where a Google employee appears to be aware.