Here's a small script for changing the flash intensity on cameras which normaly haven't such a possibility (e.g. the IXUS series...).
It's tested with a Ixus 860is (SD860) with Digic III ProperySet...
@title Flash intensity
rem Author: fe50 2008-08-19
rem Digic III propset
@param a 121 Adjust mode 0=Auto 1=Man
@default a 1
rem If Mode = 1 (Manual), the Intensity (=Output) can be 0 (low), 1 (medium) & 2 (strong flash)
@param c 141 Output [0,1,2] (if 121=1)
@default c 0
print "Set Mode to: ";a
set_prop 121 a
print "Set Output to: ";c
set_prop 141 c
print "Mode is now: ";get_prop 121
print "Output is now: ";get_prop 141
rem The flash setting is active until a camera setting or the dial mode is changed
*EDIT: For SD450 with Digic II Properties change "121" to "15" and "141" to "29" everywhere in the script !
How to use it:
-> Set up all your settings, e.g. 1/100k Ev, flash on etc.
-> load the script, leave ALT-mode with PRINT
-> position the cam, enter ALT-mode with PRINT
-> start the script with the shutter
The script waits 3 seconds, sets the flash to the lowest intensity (2nd parameter) & shoots...
Once fired, you can leave the ALT-mode and shoot "normally" with the shutter, the flash intensity setting is active until a camera setting or the dial mode is changed (e.g. switching to play mode & back to record mode)...