Hadn't seen this mentioned, but it looks like to me that with 1.3.1, if you have the phone in airplane mode and WiFi turned on, the WiFi does not cut off when the screen is locked. It's possible my phone just never worked correctly, since I had to Doctor the phone to get it to recognize the 1.3.1 update. I don't have a good Sprint signal at work so it was kind of annoying that I had to unlock the screen and wait 30 seconds to see what new e-mails popped up.

Doesn't seem like it affects battery life much more than normal use.