First, note that for these faster-than-M-mode-fastest-Tv exposures, in-camera PLAY mode EXIF view will not show the correct exposure and JPEG EXIF exposure time fields are incorrect as well. Only the resulting photo (by its lack of light) and the Canon MakerNotes section in the EXIF tag will reveal that you've actually achieved a short exposure time.
Does CHDK Tv override work in the camera's built-in Tv range? You mentioned you achieved extra long exposures, but that's a bit different from normal exposures. I'm asking this because RaduP is struggling with Tv overrides in his port, and it appears possible that some new cameras use a previously unknown propset.
Set Tv to something normal like 1/500 s and shoot (if you're in Tv mode or M mode, set in-camera Tv to something else such as 1/10 s, or if you're in a fully automatic mode, make sure you aren't in bright light and during half shoot check that the camera didn't select Tv 1/500 s).
For that photo, enable Canon builtin EXIF display in PLAY mode and check Tv. It should be 1/500 s. If it isn't, Tv override is broken (or you're doing something wrong). But if it is, Tv override works and there's something different in sx10 regarding short exposures.
You could also try shooting at 1/10000 and then check JPEG EXIF MakerNotes:ExposureTime (exiftool -MakerNotes:ExposureTime *.JPG). If that shows something close to 1/10000 (I get 1/9955 on my a570is), CHDK is working as it is on other cameras. If it doesn't, either something is different or you're not doing it right.
If makernotes exposure time indicates you successfully attempted a short exposure but your photo doesn't match that setup, it's likely that your camera just can't go that fast. The limit could still be something slightly faster than 1/3200, especially with a narrow aperture (such as F/11). Zoom could affect this limit as well.
