As a general rule, they say you should never use lower than the inverse of the focal length you're using as a shutter speed, IE
300mm stay above 1/300th. As a general rule with IS
systems (aside from the 70-200 and such advanced ones) I cut that in half and call that the safe low. You should be able to do a 1/30th sec shot at 75mm without worry, and a 300mm shot at 1/160th or 1/125th without worry. Panning helps mess this up, especially with an IS
system that doesn't know if you are panning or what (earlier generation Canon IS
systems, like yours and that on my Olympus).
I've been able to push even further down, with less reliability. If you hold steady and/or have good panning skills (when applicable), and a good shutter pull, you can go way, way low. Take extra shots, just in case.
Best I've done and still gotten razor sharpness was 1/2 second at 200mm. Took about 100 pics and maybe 2 came out.
Have fun, experiment, and don't trust the LCD
screen (if you have a digi). Take lots of pictures and only delete the painfully obvious mess-ups (chopped the plane in half, so blurry you can't tell what type of aircraft it is, etc)