Yeah, you need torx 10 and 6 to get into it, and it's just a standard 2.5" drive inside. If it's a full height drive (12.5 mm?) you're putting in then you need to bend off 4 retaining clips, but no biggy.

I have a phat one. The slim ones can only take 9.5mm high drives from what I've seen.

And does yours now have a chip of some sort?

Yeah, not sure exactly what chip but I'll probably open it up at some point to inspect the guys handy work anyway.

Think it's one of these : http://www.consolewizard.co.uk/kes/original-phat-xbox-360-jtag-rgh-flashing/53-xbox-360-phat-rgh-reset-glitch-hack.html