Update 3
I've developed an alternate version of the patch which ignores the slider when a headset (wired or bluetooth) is attached. It should be available on Preware and webOS Quick Install shortly.
Update 2
This patch is compatible with webOS 1.3.1.
Update
This patch is now available via Preware and WebOS Quick Install, thanks to dBsooner at webos-internals.
On Preware, go to: Patch -> Phone -> Close Slider to End Calls
On WebOS Quick Install, go to: Tools -> Tweaks -> KeyToss' slide to end call
Note that you'll need to restart Luna or the device after installation.
KeyToss Phone Slider Patch
This patch allows you to hang up phone calls by sliding the Pre closed.
Notes
Requires webOS 1.3.1 or webOS 1.2.1. It is NOT compatible with webOS 1.1.3.
It will hang up only if the phone is open first. If your phone is already closed, it won't interrupt your call unless you open it and then slide it shut it again.
If you are on multiple calls, this will disconnect all of them.
This works even if the phone is in card view, or running in the background.
The Pre already allows you to answer phone calls by sliding it open as of webOS 1.2.
This will probably do absolutely nothing on the Pixi and any other devices with different form factors.
If you prefer to manually install the patch, or if you want to see what the patch does, here's the patch file:
keytoss-phone-slider.patch
See VeeDubb65's instructions in this thread for manual installation.