Well let me ask you this question, is the difference in IE worth it to even bother attempting this?
I say that because, as mentioned before, I had all the versions up running against FF. I noticed almost no change in regards to the page size, text size, layout, etc. If there was any (I can't remember, as it was a few hours ago), it most definitely wasn't bad enough for myself to make a mental note of it.
My question would be why are you using the IE specific only property ZOOM on a layout in the first place as anything but a haslayout trigger? I mean, if it's written properly don't you want it the SAME size in all browsers? Or are you having the problem of size inheritance being broken in tables or something.
You'd be better off worrying about ACTUAL accessibility issues like the fixed width layout and uselessly undersized fixed metric (px) fonts... since I have to zoom in 40% to even approach legible on my desktop and the fixed width is too big for my netbook -- and it's not like zooming out is even an option.
Of course being a Wordpress based site, you're pretty well nebfered on the markup in the first place, what with the endless and pointless wrapping elements and classes for nothing... but still you could at least open it up semi-fluid and set the file sizes to something useful that would auto-expand.
Though that would point out precisely why I consider things like that rotating banner image to not be viable concepts in web design, since it can't automatically expand in height to fit different sized content... or auto enlarge on width/auto-shrink on width for different devices of differing capabilities.