Is CS2 a mistype? Do you mean CR2?
The Canon 5DII was first supported in Camera Raw 5.2, which means that you need a minimum of Photoshop/Bridge CS4 (fully updated) to open the files.
If you're using CS2, you'll need to convert the files to DNG using the latest DNG Converter, which is a free download from Adobe. Don't forget to set Compatibility to your installed version of Camera Raw.
If you've got Lightroom 2.2 or later, you can use this to convert the files instead.
Sorry, I meant CR2. My wife is the photographer...not me :)
We are using Photoshop CS5, and we have never had any issues with these files in the past. Some CR2 files from the same card are actually viewable on Bridge and in the camera.
Are you only using the one 5DII with this card?
I seem to remember reading that some Canons do some weird alternative Raw format too, but I guess that would save with a different extension.
If some images are viewable and some from the same camera are not then that certainly points to a data error. Whether this is the card or the camera it's impossible to say, but it's usually the card (or the operator) causing the problem. You might have some luck with a data recovery program. I've had mixed success in the past. Card manufacturers often provide basic software for free.