I read an article some time back where it was talking about a master violinist who had played nearly everyday since childhood. But he was noted for saying that if he missed a day of practice even with all the years of practice beforehand he could tell a difference in his playing when he started back.
I guess technology can be the same way. Like several others have said if you aren't using the tool regularly you will forget some aspects of it. But as you've seen, learning new technologies can help you learn more about ones that you already use. ex. Adding Flex to Notes, or mimicking features into your code in Notes that you learned elsewhere
Even if you forget some things you probably aren't going to forget it all. So if you use .net for a while, you aren't going to forget Notes overnight.