Hrm.. strange. IE and Firefox both will load it, then after a few seconds, it'll stop and go completely white.
I've tried clearing my cache completely, saving the flash file and playing it manually, updating flash... it just won't work, and yet every other cartoon seems to work just fine.
Edit: Just downloaded and tried it on Opera. Same exact thing. Goes 'Loading...' for a few seconds then 'Duh duh duhuhuhuhu-' and stops, the flash window goes white, and stays that way.
Try clearing your cache before checking as you may be running a locally cached version of the flash file
Edit 2: Went to
http://www.homestarrunner.kitkorp.com/sbemail55.html and it ran fine, thus sating my increasingly intense desire to watch that one e-mail.