I have a Canon 5D mark II. The weirdest thing happened today. I was shooting with a 4 GB memory in Raw and Jpeg and was only able to shoot 34 pics. I looked at my settings and couldn't figure out what I was overlooking. I usually can get close to 150 t0 200 pics. Please Help...
Did you use the CF card in your computer and either store non-picture files on it, or delete the files without emptying the trash / recycle bin when you were done? A bunch of hidden files can fill up the card's space pretty quickly (happened to me a couple weekends ago; luckily I always carry a spare).
It could also be a corrupted card, and some sectors have been marked as bad.
If you're sure there's nothing important on the card you could format it from the camera's menu, which will remove any undeleted or non-picture files.