There are tons of tools in the Internet for Windows platform to capture Flash Video (i.e. YouTube) and convert into AVI, WMV, or iPod format (QuickTime). However, there are not much choices for Mac OS X. How come? It's because the procedure is very simply so you don't need that many tools to help you out.
First, I assume you have QuickTime Pro. QuickTime Pro allows you to save any stream video into a file. QuickTime Pro is not free but I'm sure you can find other free solution very easy by using Google. So I'm not going deep into this issue.
Second, you need a plug-in to play Flash Video within QuickTime. I use an open source plug-in called Perian. I have mentioned this plug-in a couple days ago. You can get a copy of the binary from my iDisk public folder.
Third, you need to download the Flash Video from the web site. Again, tons of tutorials on the net to teach you how to figure out the URL from the HTML source code. I admit I'm lazy so I use tools to help me on that (that's what the tools are for, right?). I use Video Downloader 2.0 for Firefox. If you are not using Firefox (i.e. Safari), you can use YouTubeX.com (it only works with YouTube.com so it does not as powerful as Video Downloader 2.0)
Now, use Video Downloader 2.0 or YouTubeX.com to download the Flash Video (.flv) file to your hard drive. Then open the Flash Video file using QuickTime Pro. Use QuickTime Pro's Save as... to save the file into QuickTime (.mov) format. Done. Simple, right?