If the TubeTV download button is disabled in a YouTube video page, refresh the page (Apple+R) and hit the button while it's available.
I use TubeTV to download
I usually wait until the YouTube video loads completely before I hit TubeTV's download button, so I don't download the same bits twice. But, as of recently, I've noticed that the download button becomes disabled at some point during the video's load. I was scared for a second, and thought the days of my easy downloading are over.
After my 2-seconds panic went away, I tried refreshing the page, and the download button was enabled again. The videos are still downloaded, encoded for iPhone, and deposited into iTunes' library just fine.
Too bad TubeTV wasn't open sourced, even though looks like its author abandoned it. One day, it will stop working. Hopefully, something better will be written by then. Or YouTube will start using <video> tags and serve us the H.264 data on a silver platter. And the RIAA / MPAA will let that slide. Right.