OK, so just spent 30 minutes debugging this. I'm now using a mac as my development platform for one of the sites I support. It's worked pretty well until today when I started adding a new feature. I ran into a weird problem where POST arguments were not being made available, either in $_REQUEST or $_POST - while GET arguments were.
Thanks to this page, I was pointed in a useful directions. Turns out for some reason my version of MAMP had a max_post_size of "3200M" set in its php.ini file. That was not working too well - when I changed it to 32M, everything went back to working normally.
So, perhaps that will help someone else who's driving themselves crazy!