Having fallen in love with the 70-200 2.8 IS, I'd have to ask: why the addiction to 300mm? I ask, because there are four variants of the 70-200 (two at f/4, two at f/2.8; one of each with IS and one of each without IS) ranging from $600 to $1700. Each one seems to carry a phenomenal reputation; I have a hard time taking my 70-200 off my camera these days (though admittedly it's hurt my composition skills a bit). With the image quality any of these four can create, you can crop to match the composition of a 300mm lens fairly well, or add a 1.4x TC to get you close.
My 1D3 is in for the AF repairs, so I've been working with my 400D a lot more lately. It's a great lens on the 400D just the same. My 24-105 just seems a bit long on the 400D when I'm doing wider work, so I'm strongly leaning towards the 18-55 IS lens to replace the 400D kit lens. (I don't find the kit lens "bad", but since some of my work is done while riding a bicycle, the IS can be a real help, and the 24-105 is just a little too stiff to zoom with one finger though I'm not complaining.)