Hi Eric,
It's an incredible lens, but unless you really, really need it, I'm not sure about the 400/2.8. I guess if you are only going to have one super tele, that may be a good choice. I opted for the 300/2.8 and 500/4 combo instead... Both are smaller and lighter than the 400/2.8. For example, if using a gimbal head with the 400/2.8 (which you will probably want), you are pretty much limited to a tripod dedicated to use with just that lens. Add another $1200-1500 for that. Just something to think about and decide for yourself. If you haven't already done so, you might want to rent the 400/2.8 for a little while and shoot with it, to see what you think.
(Note: with the smaller, lighter 300mm and 500mm, you can use a gimbal attachment, like the Wimberley Sidekick, on a standard ballhead and tripod rig, so it's not a "dedicated unit" and remains usable with all your other equipment. I use the Sidekick with a Kirk BH-1 head on a Gitzo 1325 'pod. Works great.)
Regarding the 5D, I disagree about waiting for still-mythical new models. I'm sure the 5D Mk II will be great (but it's probably back on the drawing board again now that the D700 has been announced). However, the current 5D compliments your 30D very nicely. The two cameras are very similar in feel and operation. Side by side, it's obvious they are the same generation of Canon.
We can pretty much expect the new 5D will be priced $2900 to $3300. You can get the current version a whole lot less, and nearly 13MP will give you plenty of image to work with, especially compared to your current 8MP, 1.6X 30D (I'm not knocking that camera, I use two of them myself).
Yes, the 5D is slower operating. For sports, that's important if you tend to "spray and pray", turn on a high frame rate and hold the shutter release down, hoping you'll get a good shot. If, on the other hand, you time your shots, then it should be fine. It won't have as much "reach" as a cropper. Many pro sports shooters are tending to go with 1D cameras for the 1.3X crop, which some of them appreciate because it allows them to replace their 400/2.8s and 600/4s with 300/2.8s and 500/4s (none of us are getting any younger!).
But, you've got all that covered with your 30D, don't you? Maybe you'll want to upgrade that some day.
Yes, ideal would be one or the other of the 1D series cameras. Bigger, heavier, more expensive to buy and to repair, though. And, there's no option to remove the grip to make the camera more portable. But, they are workhorses and hard to beat. Still, the current 5D gives you more MP than the 1D M3, and full frame like the 1Ds M3.
Current 5D also is very good at low light, thanks to it's less crowded CMOS sensor. Who knows if the new one, which undoubtedly will have higher resolution to stay competitive, will be as good at high ISOs. I'm sure Canon will try to make it so, but whether or not they manage it remains to be seen.
Plus, we have absolutely no idea when the new 5D will appear. So you could be waiting a while. Everyone was sure it was going to be intro'd in January... Then in February... Then in April... Then possibly in July.... Now they are saying late August to September. But no one really knows for sure (except folks at Canon who aren't saying anything).
Have you considered buying a used 5D? There are some bargains around now. I prefer to buy from a reputable dealer, like B&H, Adorama or KEH.com (my second 30D came used from KEH last fall... great price, rated a "bargain" but looked near new and just lacked the batteries and accessories, which I already had).
As a nature/wildlife shooter, what about a macro lens? For use on the 30D, I'd recommend the 100/2.8. For use on the full frame 5D, I'd lean toward the 180/3.5.
Overall, I say go for it! You will have a blast, moving up from the kit/entry level glass. I think you are on a good track: Lenses before bodies. Again, probably all the lenses you are considering are available to rent, to try before you buy, if you wish.
