If it's YouTube you could use "PodTube" that will put it in the format for your ipod then you could either just use that or use "visualHub" to convert it to whatever format you want. Thats the only way i know of.
You can also go to the activity window in Safari (option+command+a) and double click the flv file (it's the largest one) to download it, then convert with iSquint![]()