Factory reset (including USB storage) prompts for password after I already typed it in I went into Settings->Privacy->Factory reset and checked "Format USB Storage". Then I pressed the button to continue and it prompted for my PIN (just numbers) as usual. Then it said it was formatting the USB storage as I told it to. And then it just shut off. So I started it back up and it prompted for a password (full keyboard; any characters you want).

I don't know what I had set that password to (I haven't used that kind of a password in a while) so now I'm stuck. Going into recovery mode does the same thing. Download mode works just fine.

So now what do I do? Is there some way I can format /data manually from my computer?

Here's some more info about my device:
Samsung Galaxy Player 4.0
Samsung Galaxy S build.prop
Rooted
Stock Gingerbread ROM, stock kernel, stock recovery
If your Recovery Mode is also password protected, you are out of luck with GUI interface from device.

Go to Download Mode and flash the ROM. This is the only solution if you don't have password.