The issue has been resolved thanks to the developer! It IS a bug in IE8/9 that I guarantee will effect others using the Traction Template (perhaps other templates from Shape 5 when using additional plugins).
After a rather lengthy investigation of the problem, I have managed to find out what makes IE 8 and IE 9 not display the background image when sigplus is enabled. Surprising as it may seem, sigplus has absolutely nothing to do with the issue. IE 8 and IE 9 seem to have a hard-wired limit on the maximum number of <link> and <style> elements they can process on the page. The exact value of the limit is not documented anywhere but seems to be around 40. Once the total number of <link> and <style> elements exceeds the value of this "magic number", the style definitions in excess are ignored. This was the reason why IE 8 and IE 9 were ignoring the background image and the page font face; both of these were defined in an inline <style> element, relatively late in the page <head> section. (IE 10 appears to work as normal in this respect.)
Once I have discovered the existence of this particularly annoying bug in IE 8 and IE 9, it was easy to find a solution: I have enabled CSS compression on your template configuration page. This merges several CSS files into a single large file and compresses the result by removing white-space and unnecessary rules. As a side-effect, it reduces the number of <link> and <style> elements in the HTML source. Once CSS compression is enabled, the site appears to work in all browsers.
I guess the most troubling/frustrating aspect of this entire issue (has been going on for some time, and reported in at least a couple of threads) is the response that the issue didn't actually exist. It does, even if it isn't seen by those with IE10 installed, and I would appreciate your time in passing it along to others moderating the forum as a likely first solution for issues with display in IE9.
It also may be a good idea to use a tool such as Browsershots.org to test the display in different browsers as whatever method is being used now didn't catch it.
Thanks again for all of your time on this issue, I know it must have been frustrating from your end as well!