Actually, in most of the Mac Browsers I am using, there is a large grey space in the middle of the header. This very well may be a validation error.
In terms of Mac IE, I personally say don’t worry too much about it. The New Macs don’t even come with IE anymore. Most Mac Users are now running Safari or Firefox. There are some who still use Mac IE, but I would suggest making sure it runs well on Camino and Opera as well… Just My Opinion…