RIM has yet to fix the issue of the Storm's screen not locking during a call. While we are waiting for the official "native" fix, Cell Avant has stepped up and released a free utility to lock the screen during the call.
The application runs in the background, and when a call is made or received, after a few seconds it locks the screen and displays an "iphone-ish" slide to unlock screen. You can even customize it with your own wallpaper.
To get a copy of the free utility visit: http://www.cellavant.com/TalkLock/Default.aspx
The site also includes installation instructions and a FAQ.