After crashing my bike last summer I developed a similar problem. Not a massive deal but really quite annoying. It turned out that I had ever so slightly bent the steering tube of the forks (I think I've explained this badly but I mean the bit of your forks that you don't see as it's inside the headtube of the frame). They were only a few mm's out but I had to get new forks in the end...
Hope your problem is something a little easier to fix.
After crashing my bike last summer I developed a similar problem. Not a massive deal but really quite annoying. It turned out that I had ever so slightly bent the steering tube of the forks (I think I've explained this badly but I mean the bit of your forks that you don't see as it's inside the headtube of the frame). They were only a few mm's out but I had to get new forks in the end...
Hope your problem is something a little easier to fix.