I found it..
The solution is this:
1. First Give your iframe the allowtransparency="true" tag.
2. Make sure the source page's content has NO background color specified... not in the page itself, and not in any stylesheet linked to the page! This will make the body color pure white.
3. Add this to your iframe tag:
Step 1 and 2 makes the iframe transparent in netscape/mozilla/firefox. As long as the page has no other background specified, the transparency works... but if you specify a background color, the transparency is lost.
Step 3 makes it work in Internet explorer, ie's filter:chroma nukes the white background. That means you must make sure no other part of your page (or at least nothing in that iframe) has pure white, or it will be made invisible. That's pretty easy, if something is even slightly differently colored (for example #FFFFFD) then it won't be transparent.
So the example code:
<iframe src="anything.htm" frameborder="No" style="FILTER: chroma(color=#FFFFFF)" allowtransparency="true">
And "anything.htm" must not have any bgcolor specified in its body tag, or in a style section, or in any CSS that it's linked to.
Hope this was helpful to someone other than me.