Okay, well, I measured my A570IS a bit.
Here's my summary of shutter speed measurement results using Fingalo/Allbest 1.00e test build 19 at shutter speed override Tv = 1/100k (= 10 us, the fastest setting, the camera is of course slower than that but I attempted to measure the actual shutter time, not changes between Tv settings).
The results are a bit more oriented to the ability to capture motion than the ability to take photos of really bright objects because of the nature of my setup. I.e. if taking photos of a very bright object, the effective shutter time may be significantly faster than listed below, especially if the object doesn't fill the entire picture (and maybe especially if you don't position the object to the center of the photo; I'm going to try to verify that some day).
Wide angle (1x zoom):
F8.0: 45 us (1/23000 s) for 10% image area, 60 us (1/16000 s) for full image area
F2.6: 360 us (1/2800 s) for 10% image area, same for full image area
Full telephoto (4x zoom):
F8.0: 210 us (1/4700 s) for 10% image area, 250 us (1/4000 s) for full image area
F5.5: 320 us (1/3100 s) for 10% image area, 360 us (1/2800 s) for full image area
Canon limits are 1/2000 seconds for F8.0 and 1/1250 seconds for the F5.5 and F2.6 measurements, but I haven't measured how long they actually are. The apertures are the minimum and maximum available aperture for each zoom position on the A570IS (I haven't experimented with CHDK aperture override yet, so if it can be pushed beyond F8.0, a faster shutter speed is to be expected as well).
The accuracy of these results must be taken with a huge grain of salt. Especially the shots taken with 100% image size suffer from significant shutter distortion. This was a "quick" first try to do this.
How did I measure?
I put a uC-controlled LED board with 14 bright red clear-lense LEDs in a dark room (lit only by the LEDs and the camera's display and power LED), programmed it to blink them really really fast, only shining one LED at a time, drawing a rectangle clock-wise and took photos.
Now, for instance, my fastest blink sequence was 77.5 us (5.53 us per LED). If all LEDs are shining in a photo, the shutter time clearly is more than the 77.5 us. If only ten are shining, shutter time would be less, maybe about 55 us.
For an ideal shutter, there should only be a number of bright LEDs and not more than one dim LED next to the first and last bright LED (the LEDs and their control are faster than 300 ns). The shutter is not ideal, and therefore there are more dim LEDs than that in some measurements, sometimes even lonely dim ones which are not next to other bright or dim LEDs.
All measurements were taken from two distances: from up close with the LEDs filling the entire width of the photo (and most of it's height as well), and from a distance with the LEDs only filling the 1/9 (roughly 10%) center part of the display (I used the camera's internal 3x3 grid for positioning).
I used ISO800, mode M, manual focus to approximately the distance of LEDs, took 5 images for each measurement using Canon's serial timer mode. I worked on the hopefully correct assumption that focus would not affect shutter speed.