I would argue this.

You NEVER know when you'll go FX - Nikon may offer an entry point in 24 months that makes it as $$ as a DX body, right?

With that said, I think a 16-35/4 and 28-300 3.5-5.6 might be a killer DX/FX travel combo.

Actually, add the Tokina 11-16 2.8 to that AND THEN it's killer :-)

Oh, and always throw in a 35 1.8 for good measure.

I own two of the 70-300 D lens. I will not sell either one. B&H has people who request that lens but do not keep a list so you just have to keep checking. Mine were bought new. I also own the 70-300VR and it is BETTER but 2x larger and heavier. I have yet to try the 28-300 but the reviews i have seen make me doubt that it is a lens I need or want to own. I intend to keep my 18-200VR as at times it is very useful. I think the 16-85VR and the 70-300VR are the best combo I have used. I am trying to get my hands on the FX lens 16-35VR as it is very highly rated and I need wide and I need the sharpness. I also lean towards say a f4 lens rather than a 2.8 since there is a distinct price and weight penalty for a field user and I rarely ever shoot at 2.8 for anything. However just the viewfinder is brighter at low light with a 1.4 or a 2.8 so that IS a factor. I also do not buy many DX only lens anymore as my D700 has cured that. I shot the D300 more than my other cameras now, the D90 is now a backup, and I shoot D700 when it is scenery or low light. I really love FX but the lens are bigger, the cameras are bigger, and the use in the field is tougher. Also the penalties for a lost or ruined camera are much greater. When D700 gets replaced I probably will not buy that camera for video.

I have been thinking of a similar switch .. but I usually shoot with 2 cameras so one with 10-24 and teh other with 28-300 is a nice combo for me. for you if you only use 1 camera at a tile then it will really depend on wether you will miss the wide end of the 28-300 if 28 is wide enough for you most of the time then go for it! 28 on dx is a nice focal lenght if that is wide enough for you .. but most people will miss the 18-28 area of a superzoom.

Unless you're moving onto FX anytime soon, I don't see a point either.

Your lenses already cover that range now, there's no point.

MY OPINION:

I'd either keep the 18-200 OR:

If you're talking the 70-300 VR or ED:
I'd sell the 18-200 and buy a 50 f/1.4 or f/1.8 and the 85 f/1.8.

If you're talking the 70-300 G or D:
I'd sell them both and buy a 50 of your choice and the 70-300 VR.

I just don't see the reason to carry a nearly two pound lens (esp. on DX) as my "every (bright) situation" lens.

I would ask sth else: what's a reason to put neon engine into nice C class Benz. For me it's not a good option, but it's Your choice.

Sticking to DX?
Keep your lenses. The 28-300 MIGHT be able to replace them, but not with a large margin. You will pay a lot to combine two lenses in one. If you itch for new lenses, sell your 18-200 and get a 17-55/2.8 instead.

I got a 10-24 and an 18-200 like you. That works well. I also use my old primes once in a while.

What are your thoughts about using a 28-300mm lens with a DX body such as a D300?

I would sell my 18-200mm and 70-300 lens and use the new lens with my 10-24mm lens.

I also hope that the next version of the D300S is a DX body.