This is only relevant to Linux 32 bit 2.68 and 2.68a. Although this bug is closed with a fix, I thought I would share my experience.
In 2.68, using cycles render, and tiles, a small black dot appears at the corner of each tile. If progressive is used, there are no black dots, but render time increases. However, when rendering from the command line, the time difference is very small. My results are below using 2.68a - build r58536:
From the command line - using tiles - 8:51.29
From the command line using progressive - 9:06:44
From within Blender using tiles - 8:58.29
From within Blender using progressive - 10:54.24
So, as you can see, the difference between rendering using tile within Blender, and progressive from the command line is only about 8 seconds.
When I tried the latest 32 bit linux build as mentioned in the fix, 2.68-r60555, the render time from within Blender using tiles went from 8:58.29 to 12:23.40. Also, the resulting render had artifacts that, at first glance appear to be related to surface normals, but I verified that the surface normals are correct for the objects.
The blend file and image have been uploaded to the bug tracker at the link posted above.
I know - I should be using 64 bit, but that is not an option for me at this time.