I've used the EF 24-105mm f/4 L IS for short periods in light to medium rain with no ill effects before. Mine has also survived collisions with football players, bumps from working in a dense crowd, and the occasional 'soft' drop from a foot or two. The thing is built like a tank.

But, as AJ Henderson points out in his answer, it is also a lens with variable internal volume as the lens is zoomed in or out. Some folks refer to such a lens as a "dust pump" because the lens must allow air, as well as the dust and moisture in the air, to vent in and out as the internal volume of the lens changes. Also, although there is a rubber O-ring where the inner barrel slides in and out of the main barrel, it is not an impenetrable barrier.

Having said that, I wouldn't use any of my "L" glass in a downpour or rain of any intensity for long periods of time without using a rain cover of some type. Weather sealed is not the same thing as waterproof!

Roger Cicala's comments re:weather sealing such as the one dpollitt cites in this answer are one reason I wouldn't trust any of my lenses in an environment where they are getting wet faster than the water can run off them. A plastic bag and some gaffer's tape are very cheap, very easy to stash in your camera bag, and very easy to use when needed.

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Update since I wrote this answer almost 3 years ago:

I've now had the "opportunity" to use this lens unprotected in heavy rain for short periods as well as for extended periods in light to moderate rain. This lens doesn't let water inside as long as the water can run off of it as fast as it is falling onto it. Ideally when I am shooting in adverse weather I wrap my gear in plastic covers, but in those instances when bad weather broke out unexpectedly and my bag might have been stored halfway across a stadium or in the equipment truck I've had no damage from water to my EF 24-105mm f/4 IS.