Took me a while to figure out how to specify a hairline border. Found these styles in BOTH the OpenOffice XLS documentation and the Microsoft XLS documentation. Enjoy!

These solid line border styles are available in BIFF3-BIFF8.

0 -> No Line
1 -> Thin
2 -> Medium
3 -> Dashed
4 -> Dotted
5 -> Thick
6 -> Double
7 -> Hair

// example cell with "hairline" top/bottom/left/right borders
$format =& $workbook->addFormat();
$format->setBorder(7);
$worksheet->write(0,0,'hairline example',$format);


These dashed line border styles are available in BIFF8.

8 -> Medium (dashed)
9 -> Thin (dash-dot)
10 -> Medium (dash-dot)
11 -> Thin (dash-dot-dot)
12 -> Medium (dash-dot-dot)
13 -> Medium (slanted dash-dot)

// example cell with "thin dash-dot" bottom/left borders
$workbook->setVersion(8);
$format =& $workbook->addFormat();
$format->setBottom(9);
$format->setLeft(9);
$worksheet->write(0,0,'hairline example',$format);

NOTE: While I assume these BIFF8 styles work fine, I could not confirm that these BIFF8 styles were set properly since OpenOffice 3.3.0 doesn't seem to support any of the dashed borders. If you have trouble, try to specify these values as hex? (ie: 10 as 0x000A)

There are several more styles you can use, for example:
3 -> dashed line
4 -> dotted lines
6 -> double border(thin)