I am about to take a trip of a lifetime to the Galapagos. I recently tried the L series 70-200 cannon but it is so heavy I hate it. I would like to find a zoom much lighter that give quality photos and has stabilization. Any suggestions?
Firstly, the 70-200 is not what I'd call a "walk around" lens It can be cumbersome and get heavy after a while especially if you have the f/2.8 version. As someone mentioned before, the f/4 version is a lot lighter.
For a regular all-rounder have you considered the following:-
EF-S 17-55mm f/2.8 IS USM
EF 24-105mm f/4 L IS USM
EF 24-70mm f/2.8 L USM
The EF-S 17-55mm really is one heck of an upgrade from the standard kit lens, and being that your 40D has a crop sensor - it will accept the EF-S lenses. It has a constant f/2.8 aperture through the whole zoom range, really fast USM, and the image stabiliser you were after... I had this lens for over a year and was consistently impressed by the image quality. It also has the UD and low-dispersion glass elements of its bigger brothers in the L series, so the quality is high.
The EF 24-105mm is a chunky option but not overly heavy. It is a full L grade lens with all the build quality that you would expect from an L. It has a constant f/4 aperture through the zoom range, which is usually more than adequate for most work, especially daytime things. Although it has a smaller aperture than the 17-55 or 24-70 it still has the image stabiliser built in to give you a couple of stops extra leaway.
The EF 24-70mm does not have IS, but few users miss it. Another chunky peice of glass with a constant f/2.8 aperture ensures good shutter speed even in low light and good DoF. It can be quite heavy after a while, but it is a seriously good bit of L series glass. Canon have, this year, released a version II of it, though as I understand this has been subject to delays and is still in short supply; but what that means is that the old version of this lens (still fantastic) should be readily available and perhaps at a better price too (though it is still expensive!).
These are the lenses I would say would be good for a "walk around" lens. The 70-200 you mention would be a telephoto zoom great for if you can't get close to your subject, but you won't so easily be able to do landscape work with it (but it is possible -- I once did a landscape at 400mm!!). I hope those options help you out...