With new iMacs comes new software. Apple has released a Boot Camp update for Thunderbolt iMacs. The small update (the download is just 638 KB) can be found via Software Update or Apple's website and offers a specific fix. According to Apple, the update "...addresses issues with Japanese and Korean keyboards on early 2011 iMac."
If you've been struggling with this issue, on the iMac you can't possibly have received yet, here's the update for you.