I'm going to try to quickly explain how this works because I'm getting a lot of questions from people wanting to make a youtube downloader in another language.
First : They keep changing it , but the solution can always be found in the source ; in the flash player section.
<div id="watch-player" class="flash-player">
fmt_url_map=
&
/fmt_url_map=(.*?)&/
35%7Chttp%3A%2F%2Fv6.lscache4.c.youtube.com%2Fvideoplayback%3Fsparams%3Did%252Cexpire%252Cip%252Cipbits%252Citag%252Calgorithm%252Cburst%252Cfactor%26algorithm%3Dthrottle-factor%26itag%3D35%26ipbits%3D8%26burst%3D40%26sver%3D3%26signature%3DA9B7F2AC733E05D7369DF0C6B2C0A802B6D9C8ED.92042D6A86879855EB9FB1B107AC53C5C5626953%26expire%3D1304308800%26key%3Dyt1%26ip%3D174.0.0.0%26factor%3D1.25%26id%3D11afdc068b4f226d%2C34%7Chttp%3A%2F%2Fv23.lscache4.c.youtube.com%2Fvideoplayback%3Fsparams%3Did%252Cexpire%252Cip%252Cipbits%252Citag%252Calgorithm%252Cburst%252Cfactor%26algorithm%3Dthrottle-factor%26itag%3D34%26ipbits%3D8%26burst%3D40%26sver%3D3%26signature%3D4CE3A0C2E09B92A066F824B006801FF41E0FA636.1701E51A241559F14A57B37F1AC9A8E0B1C7D9DA%26expire%3D1304308800%26key%3Dyt1%26ip%3D174.0.0.0%26factor%3D1.25%26id%3D11afdc068b4f226d%2C18%7Chttp%3A%2F%2Fv9.lscache2.c.youtube.com%2Fvideoplayback%3Fsparams%3Did%252Cexpire%252Cip%252Cipbits%252Citag%252Calgorithm%252Cburst%252Cfactor%26algorithm%3Dthrottle-factor%26itag%3D18%26ipbits%3D8%26burst%3D40%26sver%3D3%26signature%3D84306C633F7DC4FD7D0751304B7EC81260254E3D.59959ED5D66A41F49C53AC860EF2C025121C64E9%26expire%3D1304308800%26key%3Dyt1%26ip%3D174.0.0.0%26factor%3D1.25%26id%3D11afdc068b4f226d%2C5%7Chttp%3A%2F%2Fv16.lscache1.c.youtube.com%2Fvideoplayback%3Fsparams%3Did%252Cexpire%252Cip%252Cipbits%252Citag%252Calgorithm%252Cburst%252Cfactor%26algorithm%3Dthrottle-factor%26itag%3D5%26ipbits%3D8%26burst%3D40%26sver%3D3%26signature%3D0F05799111FFBCC21278F2D345FD4F331DA61BB8.94D4070F6869448CAED16803C48CB2EDF5F44F3B%26expire%3D1304308800%26key%3Dyt1%26ip%3D174.0.0.0%26factor%3D1.25%26id%3D11afdc068b4f226d
Crazy ;/ I know.
After you run it through urldecode it will look something like this:
35|http://v6.lscache4.c.youtube.com/videoplayback?sparams=id%2Cexpire%2Cip%2Cipbits%2Citag%2Calgorithm%2Cburst%2Cfactor&algorithm=throttle-factor&itag=35&ipbits=8&burst=40&sver=3&signature=A9B7F2AC733E05D7369DF0C6B2C0A802B6D9C8ED.92042D6A86879855EB9FB1B107AC53C5C5626953&expire=1304308800&key=yt1&ip=174.0.0.0&factor=1.25&id=11afdc068b4f226d,34|http://v23.lscache4.c.youtube.com/videoplayback?sparams=id%2Cexpire%2Cip%2Cipbits%2Citag%2Calgorithm%2Cburst%2Cfactor&algorithm=throttle-factor&itag=34&ipbits=8&burst=40&sver=3&signature=4CE3A0C2E09B92A066F824B006801FF41E0FA636.1701E51A241559F14A57B37F1AC9A8E0B1C7D9DA&expire=1304308800&key=yt1&ip=174.0.0.0&factor=1.25&id=11afdc068b4f226d,18|http://v9.lscache2.c.youtube.com/videoplayback?sparams=id%2Cexpire%2Cip%2Cipbits%2Citag%2Calgorithm%2Cburst%2Cfactor&algorithm=throttle-factor&itag=18&ipbits=8&burst=40&sver=3&signature=84306C633F7DC4FD7D0751304B7EC81260254E3D.59959ED5D66A41F49C53AC860EF2C025121C64E9&expire=1304308800&key=yt1&ip=174.0.0.0&factor=1.25&id=11afdc068b4f226d,5|http://v16.lscache1.c.youtube.com/videoplayback?sparams=id%2Cexpire%2Cip%2Cipbits%2Citag%2Calgorithm%2Cburst%2Cfactor&algorithm=throttle-factor&itag=5&ipbits=8&burst=40&sver=3&signature=0F05799111FFBCC21278F2D345FD4F331DA61BB8.94D4070F6869448CAED16803C48CB2EDF5F44F3B&expire=1304308800&key=yt1&ip=174.0.0.0&factor=1.25&id=11afdc068b4f226d
Still pretty crazy looking, but if you study it you will see a patern.