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Finally something interesting.. I am not an Electical/Electronics Eng, but here's my .02.
I own the ST-E2 and 580EX. ST-E2 AF beam sucks big time comparing to 580EX's, I did a shoot on a beach in the evening once, the ST-E2's beam won't go farther than 15m (if I remember correctly, the manual of ST-E2 may tell you the range of the AF assist light). Another thing about ST-E2 is the battery-2CR5..it doesn't last long, since every time you fire it, it fires the flash too (yes, ST-E2 has a small flash tube inside). 580EX is better, but it's not like we want to have the bulky hotshoe flash on our camera all the time..
Now, there are two ways to do it:
1) Get an IR flashlight, and hook it up to a pressure switch, turn on the IR when you are focusing. I need to do more test before I can confirm Canon's AF works with the IR LED I have..
2) Figuring out the pinout of the Canon hotshoe. In other words, once you identify the + and - for controlling the AF assist light, you can simply hook it up to a device with external power and a powerful LED (say, a red CREE LED @ 700mA + aspherical lens will blow the 580EX's AF LED away). Using an op amp like mpmendenhall said, will be the easiest way to implement it.
So, for 2) we have to identify the pins..here's a site which may be useful: http://188.8.131.52/e/fo/