It's a bug in combination with Anti Aliasing, when you turn AA off it will be gone. It has something to do with the way AA works and you can see it in other games, too.
You could try and use a bigger 3D resolution instead, the border smoothing will not be as good, but texture filtering is improved further and especially the foliage benefits from it. Maybe couple it with SMAA post processing (for example PPAA=3; PPAA_Level=1; in if.cfg).
150% 3D resolution runs about as good in Arma 2 as AA on low for me. That's in 1920x1200 with a 5870, so you should be fine.
And if you don't mind testing, tell me how 200% works with your card, maybe even with AA low added or SMAA, I might want to do the upgrade