Nimbus L&F: There are no grid lines in a JTable.
This is observed on Nimbus L&F on all OSes, with the current 6u4b02 pit build.
By default a JTable in Nimbus L&F does not have any grid lines. There is no other way to override this behavior.
In the attached test, I have a JTable with the horizontal and vertical grid lines propertiers both set to true as below:
....
table.setShowHorizontalLines(true);
table.setShowVerticalLines(true);
....
However when the UI comes up in Nimbus L&F, there are no grid lines appearing. The synth xml for nimbus seems to have set this boolean values to false and setting the properties programmatiocally is not overriding this behavior.

Sample code is attached for reference.