This just started to happen and I'm not sure why or is it just a Solidworks 2009 thing. When in an assembly I change a parts transparence and then select edit so I can modify the part. Normally I do this so I can add screw holes and get to the part under the one being edited. After I do a set of holes and exit the hole wizard with the idea of starting a new set of holes in the same part the part changes transparency back to solid. The only way to get the part back to transparent is to exit the edit mode which switches the part back to transparent all by itself and then start a new edit.