Turning Wi-fi off when your phone goes to sleep can be done by going to wi-fi settings then choosing Advanced from the menu button and adjusting the Wifi sleep policy.
Here are some updates on my experience with this black screen issue:
Setting the wifi sleep policy to turn off when the phone sleeps consistently prevents the black screen issue for me.
However, since I often need wifi on all the time it is not a good workaround for me. So far I have noticed that on occasion I can keep wifi always on for a day or so with no black screen but then somehow the problem reappears and then I go back to almost consistently having the black screen problem whenever the phone goes to sleep. I have not yet been able to determine what conditions cause the problem to go away or what causes it to come back but I am still investigating.
When the problem does occur I have not been able to get TheSarge's non-reboot fix to work (the phone never flashes after holding the power button for over 2 min), however what I have found works for me is
1) Hold down the power button for 2 seconds
2) Open the slider
3) Press the direction pad down at least 4 times
4) Press OK
5) Press OK again
After a couple of seconds the phone should reboot.
This helps me avoid having to do a battery-pull which is a pain for me since I have case for my phone that gets looser the more you take it off and put it on.
Also, as Karendar mentioned a few posts back, i've confirmed that if you shine a flash light on the black screen you can actually see the lock screen.