I can see both sides of this debate. This is yet another decision one has to make for himself.
But, if you think about it, it really is "simple". Does it bring glory to God? No. And whether right, wrong, or indifferent, if it is percieved as wrong then in the minds of the onlookers it is wrong (which really is where your witness comes from right?). And I've found out that it's really not a big deal anyway. Diet Coke tastes just fine to me.
I personally don't have a problem with it, I mean the US is the most sober country in the world, of the ones that can afford it anyway....