I just read a thread on Linked In about modeling tools in Agile, where Douglas Swartz recommended three tools to capture your diagrams from a white board, with good quality.
Haven’t tried any of them yet, but I though I would share the info. It seems the first one is now owned by PolyVision and costs some money (I couldn’t get the exact number, but I haven’t looked very much). The two others are shareware. Clearboard is a available for less than 100 €, whereas PhotoNote seems to cost only 50$ (35€).