Fabio Sonnati of Flash Video Factory posted several notes and some demos of his tweaks for Flash H.264 on his blog, Flash Video Optimization and Tools. He got great results on the 720p High Definition (1280x720x25Fps) plus HE AACv2 audio @ 500Kbit/s (for full-screen, double click on the movie) and for the YouTube size at 124 Kbit/s. YouTube encodes at around 350 for its lo-res version.
Fabio is using a mix of (parameters that hopefully CS4 will have) "Ffmpeg, x264, Mencoder and Nero AAC. Here some parameters used: 5 reference frames, 5 B-frames, authomatic B-Frames, B-pyramid enabled, adaptive macroblock type, advanced Trellis on, Subq=7, advanced exagon search, deblocking filter with custom alpha e beta parameter, three pass encoding...".
Update: Adobe has an HD Gallery to showcase other work, and CamcorderInfo posted Round-up: Watch HD video. Sorry, but "Operation MySpace" leaves a bad taste.
Don't forget this issue according to Adobe: "One important thing about playing an H.264 video file as progressive download is that the moov atom needs to be located at the beginning of the file, or else the entire file will have to be downloaded before it begins playing." You can fix this with QTIndexSwapper (an Adobe AIR app by Renaun Erickson).
...Later Fabio presented at Adobe Max 2009,