I managed to replicate the mesh top by printing on glass at 70C with no print cooling fan:

2014-08-03 18.52.23.jpg

Then I re-ran the print on cold painter's tape, with a cooling fan, and the holes in the mesh are gone:

2014-08-03 18.52.38.jpg

With the hot bed and no cooling fan, the first layer of bridging drooped down into the cavities in the knob, so it didn't provide any support to the top layers. Without any support, the top layers didn't squish, and they didn't spread out wide enough to touch, leaving gaps:

2014-08-03 18.54.31.jpg

Otherwise, there doesn't appear to be too much difference between the two prints:

2014-08-03 18.53.01.jpg2014-08-03 18.53.07.jpg