I have the f4 non IS and will be upgrading to the 2.8 IS shortly.
I borrowed a friend's 2.8 IS to shoot the Grand Final last month.. it's another world, the f4 is a good lens, but that extra stop makes a huge difference. The limited depth of field makes a big difference in the real world, and the AF is better of course.
The IS is also good though. I'm not bad at handholding, but I still can't really get away with 1/30th at 200mm. Can with IS though.
So, as always, it depends what you shoot, and how you like to shoot it. Personally, I like 2.8 and IS, so I'll be buying both. Sure it's heavier, but my 5D and grip are heavier than my 400D. If you need it, you won't mind.
Just a thought, isn't it just the IS versions that are weather sealed? So the f4 IS will be, and the 2.8 non IS won't, if that matters to you.