If I could reproduce the problem I'm fairly sure I could figure out the cause by process of elimination and chopping things out until it goes away then putting things back until it happens again. But I have never been able to reproduce the problem on any of my machines therefore I have never been able to come to a conclusion about the cause. Even those who have experienced the problem report that it only happens sometimes on some machines.
From my point of view it is not conclusive that the problem is caused by CSS though that might be the cause, it seems just as possible that it is caused by javascript.
It can also possibly have other contributing factors such as browser plugins and toolbars and anti-virus software. I would see first if you find the same problem using other machines. While it is a frustrating problem, if you find it doesn't really happen on most machines then at least it is less severe of a concern.
Does it happen just browsing the site or are there other relevant steps such as using the wysiwyg editor? I have experienced some occasional browser errors or bad performance in the editors when SCAYT (Spell Check as you type) is enabled which it is by default. If that were identified as the problem you could disable it by configuration.
If I had a machine that would produce problem repeatedly the steps I would try to solve it are as mentioned above chopping things out.
My first suspect would be the script that Artisteer includes because I know they do some funky things for layout with IE where they re-write much of the page html from javascript using table layout. So the first thing I would chop out is the script from layout.master:
<portal:SkinFolderScript ID="sfs1" runat="server" ScriptFileName="script.js" />
If removing that solves it and the site still renders correctly I'd just leave that out.
You might also try removing any IE compatibility meta elements such as this:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE8" />
if it exists and see if that is a factor. Or if you don't have that try adding it.
If the problem persists after that I would try commenting things out in the CSS until it goes away. I'd start with the things other users have mentioned such as background repeat. I would probably also do some googling for "css crashes IE8" and that may inform my strategy in terms of ideas of what I should try commenting out in the CSS.
Is your site at a public url where I or others could try it with IE 8/9?
I wish I could be more help and I totally get your frustration but I don't think anyone has led you down a bad path. If valid CSS can crash IE it is clearly a bug in IE, the only sense that it can be considered a bug in Artisteer or the exported design is in the sense that they have not avoided the browser bug. The only path I want to lead you down is the path of success but I cannot promise that everything will always be perfect because the world is not perfect and technology is not perfect and I am not perfect.
Hope that helps,
Joe