Swfresize works with SWFObject and takes just two lines of code to implement. It keeps the aspect ratio of your SWF file and stops off-stage assets from appearing.
Basically, it sounds like a better alternative to setting your movie to have 100% width and height.
Although the link to the swfresize page on James's blog doesn't seem to be working at the moment, the download link for the swfresize.zip file does work.
See swfresize on OSFlash.