I've finally decided I might as well mention this... I'd been saving up my karma points for something seriously curly :p.
For browsing these forums I'm usually using Mozilla 1.x on a Windows box. When I do, stuff in [PHP] tags is double-spaced, quite a bit harder to read. I happened to stop by via IE at one point, and - nope, no double-spacing, so presumably it's unintentional (though what would be the intention?).
This only happens with [PHP], not [CODE], which remains single spaced.
Haven't seen it via a Linux machine, so can't give that comparison; from what I can make out by looking at a page in binary, however, each line ends with a carriage return character (0x0d) and a <br /> break, and Mozilla's going ahead and rendering both as linebreaks. (Just checked: in [CODE] blocks, carriage return - linefeed pairs are being used and no <br /> tags.)