1) A PTC acting as a fuse in series to prevent a short from cooking off my USB port.
This makes people feel better but in practice it doesn't help as much as you think. PTC's are very "loose" devices and a 500mA PTC won't trip until about 1A or so. The Bourns MF-MSMF050 says it will take 0.15 seconds to trip at 8A -- that's more than enough time to cook the USB port.
Besides, computers are supposed to have built-in current monitoring and protection such that if a USB device does draw excessive current, the USB port is shut off and a dialog box appears telling you "Your USB device is malfunctioning, etc. etc."
2) A cap bridging 5V and GND to smooth out the load.
That's fine too, but don't go too high else the inrush current that occurs when you first plug in might just look like an overcurrent event to the PC.
Of bigger concern is that you are not supposed to draw more than 100mA from a USB port when you plug in, until the "USB chip" on your board (none for yours, but PC's expect that there is one) enumerates and identifies the board as needing 500mA. Again, you might get the nasty dialog message telling you you're drawing excessive current.
Ladyada has pretty good technical discussion on this: http://www.ladyada.net/make/mintyboost/icharge.html
The Rugged Audio Shield
: Line In, Mic In, Headphone Out, microSD socket, potentiometer, play/record WAV files