X10 Filter Solution
I found a 40mm-49mm step-up ring by S&W Photo Co on Ebay which fits on the X10 and produces no discernible additional vignetting. This is a solid ring so that with a filter in place it will provide complete protection for the lens. While it won't accept the X10 lens cap, it does fit in the dedicated X10 camera case so the lens cap is not really needed. The last time I checked there were still some available of Ebay.