Another option you can do, I do this, is when you are on the phone call, press the home button and that will keep the phone call on, but make the phone app go off the screen. I do that because I have a skin/phone cover that makes the buttons kind of hard to press, so I downloaded Volume Control by RevoSolutionary Development and I open that and use the slider to change the volume. If the other party hangs up, obviously your phone will hang up. If you are done, you just press the End call or you can press Phone again and then End Call on the phone app.
*NOTE* I've only done this with the phone plugged in and I'm on a call right now, so I don't know if the phone will sleep and then you have to press End call to wake it up, etc. I think someone else said you can press the Trackball to wake it up while on a call, then you won't hang up on accident.
Yes, the trackball will wake it up again and it doesn't need a swipe to unlock either. I just tested.