It is not in compatibility mode as far as I know but I have uninstalled and reinstalled IE9 now and it seems to have gone away.
Here is my page anyway just for you to confirm. I have deliberately left the terrible text shadow filter on to show you what I am struggling with in IE9, can you suggest an alternative to this. I am using filters for gradients, dropshadow and opacity wherever necessary because as an e-commerce business, many of our customers will have IE7,8,9.
This has been an interesting journey learning to use various CSS3 generators and using a combination of filters and CSS3 rules. I even went to the trouble of creating an SVG image background conditionally applied to IE9 to avoid the square gradient box showing through the rounded corners but I am a bit fed up of how long it takes so I would appreciate it if you could enlighten me on the text shadow issue.