The longest shutter speed available for a Powershot G in Av mode is 1 second. There, I've said it. OK, so I'm not 100% sure about this. The G1 or G2 might be different but my G3, G6, G9 and G12 all have this problem -- or feature, depending on how you look at it -- and I've read enough questions and complaints to feel sure that the G5, G7, G10 and G11 are programmed with the same limit. The 1 second limit is not a bug because all the G series have it. Although not a particularly serious problem, the 1 second exposure limit is unexpected -- especially since I typically forget!
Perhaps the 1 second limit is Canon's attempt to save us from ourselves because, of course, we could not hand hold the camera for that duration. But we also could not hand hold the camera for 1/2 second or, after several coffees, even 1/30 second. Plus, the little jiggly camera icon comes on to warn us at slow shutter speeds. Besides, the shutter speed can be set for more than 1 second in Tv or, or course, M modes. Therefore, I suspect that the 1 second limit is an arbitrary one or perhaps a programming convenience.
The vast majority of the time that the 1 second limit in Av mode becomes a problem to me is when I'm making a High Dynamic Range (HDR) photo. To make an HDR photo, I typically mount the camera on a tripod, set the lowest ISO, select Av mode and set a mid-range f-stop. I then use the auto bracketing feature of the camera. The G series auto bracking can be set for 3 shots at +/- 2 stops. When the camera is in Av mode, the "stops" are adjusted by changing shutter speed. As an example, if the "correct" exposure is f5 at 1/60 second then the auto bracketing process produces an underexposed shot at f5, 1/240 and an overexposed shot at f5, 1/15 second. Most of the time, this works out fine; however, if the base case exposure is, say 1/2 second, then the overexposed shot is at 1 second instead of the expected 2 seconds. If the base case exposure is at 1 second then the "overexposed" shot is also taken at 1 second which is not useful.
For purposes of bracketing in Tv mode, remember that the (most recent) G series really has only 4 stops of aperture exposure bracketing: f2.8, f4, f5.6 and f8. Not all of those apertures are available at all zoom settings. Also, for purposes of creating an HDR image, variations in aperture produce variations in focus and depth of field.
As a result of the 1 second exposure limit in Av mode, long exposure HDR sets must be done in manual mode.
Now I can use this post as a reference for anticipated future complaints and explanations.