For portability, relative paths might help you out better.
Don't forget, also, that if you include() files by URL, you can't include and parse PHP scripts. If the only purpose for include()'ing a file is to output it, perhaps you should loook at [man]fpassthru[/man], because surely it must be more efficient than include() (which tries to evaluate the document and parse any PHP code found).
EDIT: Also, here's an explanation about the "dots" in a normal filesystem (not sure about MAC's and such, but I know this holds true for PC's and *nix):
'.' = Current directory
'..' = Parent directory (one directory up)
'../../' = Parent directory of parent directory (one directory before one directory up)
'../../../' = etc.