Save as... doesn't seem to work (Mac)
Neither do 'export tilemap as xml/plain text'.
To reproduce, use an existing file, then all of these options do pop up a dialog (all three suggest "Copy of <current name>.pyxel" as filename) but upon clicking OK none of them seem to actually do anything.
OSX 10.8.4 / Air 220.127.116.110
(Related: where can someone see the PyxelEdit version/build number they're using?)