Hi Ian,
I suggest the LowePro Apex 110 AW. It contains two small pockets in the lid. One large pocket, which is able to divide in two, any way you want, a larger divider which isn't removable which creates a medium sized pocket, with a smaller one inside on the long side. Furthermore the bag has a small netpocket on one of the shorter sides (large pocket). You can carry it either by a adjustable shoulder/neck strap, or in the belt, by drawing the belt through a "tunnel", on the back of it. You can't unhook it without removing the belt first. Oh, and the lid opens away from you, so you can easily grab your camera while having it secure and a good view of the contents.
I bought it, and besides the SX40, put on the side, you can carry the charger, two extra SD cards in small pockets on the inside of the lid, the little start up guide that came with the camera, and by using the divider, I can fit my small older S80 beside it, or like I currently have, a small cleaning brush. Other than that, there's a small net pocket where you can put some smaller things, and on that side it's also a soft microfibre cleaning cloth. If you'd like to carry some lenses this should also be possible, instead of the extra camera or even if you would like, place the devider at the bottom and put your camera on top of it. I haven't tried this myself, though. Putting the divider a little bit from the bottom will add extra protection though. I read a review on the Amazon's site in G.B. where a guy had fallen down a stair with his camera in this bag, and it hadn't taken any damage at all. (Haven't tried this myself, either 
)
Besides being built in a material that ought to withstand some rain without any problem, it also has a rain cover, in an outside pocket. See the demonstation at the link below to see this and more.
I don't have a picture of it for the moment, but check this link:
http://www.youtube.com/watch?v=90ItfWX4OOk
, which shows a demonstration of the bag. Unfortunately it's in Italian, but the function and pockets shows really well.
I think it is really good.
Other than this bag, I know some people have chosen the LowePro Rezo 110 AW, and some who want it even lighter, have selected the LowePro Quick Case 100. The former is quite like the Apex, a little different in pockets and a little smaller I think, while the Quick Case 100 just takes the camera, and perhaps a memory card. The last you carry with a neck strap.
Good luck with your SX40. 
/Tobias
