Zune Thoughts mentioned this IE plugin that adds a one-button file converter to your browser with a single purpose: convert .flv files to Zune compatible .wmv files. .flv files are typically used by YouTube and Google Video, so this could come in handy to carry those videos on your Zune.
I ran the quick install this morning on IE 7 within Vista and hit up our YouTube channel for the shortest video (our "iTunes movies on a UMPC" production). I clicked the "Zune It" button in my IE tool bar and saw that the YouTube video was then downloaded automatically to my "Videos" folder. The plugin install will actually create subfolders in your "Videos" folder: one each for YouTube, Google Video and SoapBox.
A few minutes later, thanks to the ffdshow video decoder that the plugin uses, I had a .wmv file in the YouTube subfolder; a simple addition of that folder to my Zune software library provided auto-synchronization. The entire process ran seamlessly in the background and the 11 minute video was downloaded and converted in about 8 minutes!