penders - 10:41 am on Nov 11, 2011 (gmt 0)
Changing the background-color of the iframe'd document, as Fotiman suggests, to match its surroundings should work. However, I'm curious as to why my suggestion - to add allowTransparency="true" to the iframe element itself does not appear to work for you? The background of the iframe'd document must not be set in this case - which is how you appear to have it. I tried this on your live site using IE8's developer tools and this worked for me.
Aside: the allowTransparency method (although non-standard) does have the advantage of allowing the iframe to match any background set in the parent document, which would otherwise be very difficult if a patterned background had been used for instance.
Also I'm using XHTML.
You have an XHTML DOCTYPE but your pages are served as text/html, so this makes no difference.
Maybe I can solve the problem by not using iframe.
Well, an iframe is unnecessary IMO and I'm sure would solve the problem since it would seem to be directly related to the iframe. A non-iframe solution could in fact be more tidy and easier to implement.