You can pop in your Flash alternative as the last item within the <video>
tag, and it’ll play if the HTML5 video doesn’t.
See Mark Pilgrim’s example, as it’s comprehensive and regularly updated: http://diveintohtml5.ep.io/video.html#example
To summarise it:
<video>
<!-- HTML5 video -->
<source src="video.webm" type='video/webm; codecs="vp8, vorbis"'>
<source src="video.ogv" type='video/ogg; codecs="theora, vorbis"'>
<source src="video.mp4">
<!-- Flash player fallback for user agents that don’t support HTML5 video -->
<!-- All user agents that don’t understand the <video> tag, or don’t support
the video formats you’ve provided, will show this instead. Even IE 6. -->
<object width="320" height="240" type="application/x-shockwave-flash"
data="flowplayer-3.2.1.swf">
<param name="movie" value="flowplayer-3.2.1.swf">
<param name="flashvars" value='config={"clip": {"url": "video.mp4", "autoPlay":false, "autoBuffering":true}}'>
</object>
</video>