Get a 70-200/4 IS.... It's a great lens, very sharp, fast focusing, excellent color and L-series build, sealing and durability. You'll want to use "Mode 2" IS for those panned shots. Don't worry about possible future FF... if you get one, you can continue to use the 70-200 and simply get a longer lens to go with it. Or just stick with crop cameras (which have some advantages over FF unless you are regularly making really big prints or doing really high ISO work).
Compared to the 70-200/2.8 IS II, this would leave considerable money in the bank or to use toward other lenses. For example, you might want to check out a Canon 10-22mm to replace the Sigma. Looking at your Flickr, it appears you are using wide angle a lot, too.
I don't know how much you use the 18-55 or 50/1.8, but there are upgrade possibilities for those, too. And I normally would say "lenses before camera bodies", but you may want to look into a 70D, too, once they're available. Or a 7D. Higher frame rate for those panned shots. Also, at least in the 7D, lightning fast AF for action shooting (70D looks to be similar, but who knows).
EDIT: Someone suggested, and I agree, you might want to consider a quality Neutral Density filter. Some of the slower shutter speeds for those panned shots, you are already at ISO 100 and forced to use dangerously small apertures. On your 18MP camera, any aperture smaller than f7.1 starts to lose some fine detail to diffraction. Probably not noticeable at f8, and may be fine in all but the largest prints at f11... but at f16 an any smaller, you'll start to notice it more and more.