Right. The chip on the Arduino board comes with a bootloader pre-burned, allowing you to upload programs to it without an external programmer. All you need is a USB cable to use the board. It should work fine from a Mac (though version 0005 of the software might not work on 10.3.9; you'll need to download version 0004 or wait for 0006). You can remove the chip and put it in another circuit.
In theory, it's possible to use the Arduino as a substitute for an STK500, but we haven't gotten it to work yet (mostly because no one's had time to work on it).