Is the youtube export quality improved in iMovie 09?
iMovie has little to do with the end quality of files other than determining the size/duration/quality of the files sent to YouTube. All files sent to YouTube are converted to FLV by Youtube. This conversion by Youtube itself determines the final resolution/quality of your files. If quality is what you seek, then it is probably best to create your own website and post your video at the resolution and level of quality which you can manage yourself. YouTube and similar sites aim for compatibility of viewing -- not quality.
Is 640x360 still the maximum size for direct upload to youtube?
I don't believe there are restrictions on resolution -- just duration (10 minutes) and file size (1 GB). Whatever you upload will be re-compressed to fit in the 1 GB file size limit. Higher quality means shorter times. Larger files usually mean more compression to fit it within the YouTube file size restriction. Your best bet is to review the Help resources area at Youtube.
You can send to YouTube at a max of 1280x720 in h.264, with a max of 1GB and 10 minutes.
The SHARE/YouTube Presets max out at 640x480, so you will want to use Share/Using QuickTIme to get the 1280x720.
Then you will need to upload via the YouTube Web Page.
[Here is a sample of what it looks like in 1280x720|http://www.youtube.com/watch?v=-WQOcu_EEPw]
Be sure to click "Watch in HD" at the lower right of the YouTube screen.