RC means Release Candidate. In case you don't know, a candidate is something or someone not yet selected or official for the purpose intended.
Now, I'm not surprised that the demo page on Dynamic Drive doesn't work in IE 8 RC1, no attempt has been made for it to. I am a little surprised that compatibility mode doesn't fix it though. That's precisely what compatibility mode is for. If it doesn't work, while clearly the script works in IE 7, then compatibility mode is broken in IE 8, or you have applied it incorrectly.
If you want more help:
Please post a link to the page on your site that contains the problematic code so we can check it out.
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