Not a chance of replacing the 28 - 300mm IS L, it was only released in 2004 replacing the 35 - 350mm version, and is a good performer. How on earth do you think they could improve it realistically?
IS is 3rd generation, and the amount of glass required and pro spec body means it's going to weigh quite a bit.
There are quite a few lenses I'd like to see replaced before this one, the 24 - 70mm f/2.8 L and a new decent performing wide angle too!
I don't know how they could realistically improve it really. I rented one for 30 days from lensrentals.com for my honeymoon to the Canadian Rockies and loved it, in spite of the weight. I did find myself wishing many times that it was faster.
I am a simi-pro photographer and am looking into purchasing a good tele-zoom.
I currently own the EF 50mm f/1.2L USM, EF 16-35mm f/2.8L II USM and the EF 70-200mm f/4L IS USM. These lenses are mounted on a full frame 5d (mk1) and the 1.6 cropped Xsi. (I'm waiting for the 5D III to be released before I upgrade bodies)
I found the 28-300 L to be a great "all around" lens. I loved the reach of that 300mm (being that I was used to a reach of only 200) and 28mm was wide enough I didn't have to carry additional lenses. I am seriously concidering the new EF 70-200mm f/2.8L IS II USM... I just wish it was a little wider to keep from having to swap out lenses mid shoot. I could care less about the yet to be released 70-300 because of the slow speed.
So... I was in "wishfull thinking land" hoping that there might be a faster version of the 28-300 in the works. But I guess that's just a pipe dream. I'll probably end up buying the new 70-200 with an extender... I'll still need something for that mid-range though. I guess I'll worry about that if and when they release a new 24-70.
As for your decent wide angle hopes... I know it's only 16mm, but I really enjoy my EF 16-35mm f/2.8L II USM.
Keep on clicking...