This is usually a mip-mapping issue, what is happening is that your graphics are using the smaller mip-maps and staying on them until you get very close. It was supposed to have been fixed by the patch, but it doesn't seem to have been as far as my experience of the patched version of FSX gos. But...
If you are having problems with this, turning mip-mapping off or messing around with some of the FSX anti-aliasing options should sort it.
BTW, just in case if you don't know what mip-mapping is: Mip-mapping is a method to speed up your computer by using multiple textures at different sizes. How this works is the original texture file of (for example) an aeroplane in the sim, will actually have about four versions of the texture in different sizes, and when your plane is very far away, it will use the small one to display your aeroplane's textures. In this way it uses a bit less memory and processing power to display it, and it (theoretically) should look better at a distance without everything looking razor sharp, as it would do up close, which is the main reason for it. Initially, a mip-mapped image will take up more RAM, because it loads several versions of the same texture, but once they are in, it will speed things up if you have enough RAM.
Chock