I see You're the person that asked about the gasket retrofitting? That would've been great to have, even unnoficially, or even implying voiding the warranty.
No, Bryan asked about the retrofitting. I posed the original question to Mr. Westfall about sealing on the 400/5.6L.
At the moment i have a 7D. I thought about a 2nd body, a 5D Mk.II, but resisted the temptation and invested in lenses instead. I'll skip the 5D Mk.II and get a 1D when the time is right.
In fact i just got the 400mm, lack of full weather sealing and all. I thought the lack of IS would make it unusable (since it's a 640mm equivalent on APS-C), but it's not that bad. I'll just have to get some sort of weather protection when using it outdoors, other than this issue, my first impressions are positive. The 100-400mm was too cumbersome, i would be using it at 400mm mostly, and i got the 70-200mm range covered already. I'm not sure the 400mm f4.0 DO is fully weather sealed, but at the price Canon is charging for the DO you can make a bit of extra effort and go straight to the 500mm which is a far better investment (and would be even better if it included a chiropractor ).
Almost always good advice to invest in lenses more than bodies. The 400mm f/4 DO is fully weather-sealed, yes, but I agree - the reduced contrast with DO is a turn-off, especially in that price bracket.
The 100-400mm is bulky, but one thing I like over the 400/5.6 is that the 100-400mm stores more compactly - I frequently go out with the 7D + 100-400mm in a Toploader Pro 75 AW and it's a perfect fit; the 400/5.6 is too long for that.