Matt, I have I Movie HD on my mac and had the same problem the first time I tried to put a video on You tube. It could be that you saved it under the wrong file extension as you tube only excepts certain flies such as (.mov)
Try this, reopen it in Imovie HD
Click on File
Click on Export
Then under the Quciktime Icon Choose "Compress movie for Expert Settings" this will allow you to choose the file extension and keep the same video quality. Choose ".mov" Once compressed try uploading that to you tube. It may take a while but it should work.
Let me know how it turns out