Sure... if you run 1440x900 on 2880x1800, every pixel = mapped to a square block of 4 pixels
It's basically the same as having a 1440x900 monitor where each pixel is a block of 4 uniform pixels.
I imagine that some monitors might introduce some extra input lag or something with a non-native resolution (I'm not sure on that), but in terms of pure image quality, there's no issue.
Now, does a 2880x1800 picture look better than a 1440x900 picture? Sure, it has 4x the visual detail (which is a HUGE difference). However, any blurriness won't be due to the monitor displaying a non-native resolution, it will just be because the picture is lower resolution and has less detail.