I've taken a pass at showcase to update the tabset component css for better IE 9 support. I wasn't able to reproduce the tabset issue as seen on the attached screen shot using the IE9 default rendering mode (things did go wonky with IE 9 compatibility).
Philip can you do a click though on showcase to see if there are any other glaring issues on IE 9. I think this can be closed but I'll leave that up to you.
Screen shot showing padding between tabs.