Even with the keypad lock activated the middle button can get pushed unlocking it.. and then all kinds of thing go on including unwanted phonecalls. Here is something that helps a lot. Buy a silicone case and cut out the part of the case over the center button. The rubber ring remaining over the navigation ring sits up high enough, and thus recesses the center button enough to make pushing the center button that much more difficult. It works for me. Here's a link to a case that already has the center cut out. Good luck.
http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWAX:IT