[eluser]stevepaperjam[/eluser]
The only thing I can think of is to cut it down to a base MAMP install as well, and try that with a fresh CI download, and work up from there. It should work - I've done it myself recently when I upgraded to 1.7.
The bit where you say you've got your base path pointing to a folder on your desktop shouldn't in theory break it. Shouldn't ;-)
If we're getting desperate, is there anything else running that might cause oddness, like the standard OS X Apache install?