Ben Heck has taken a sledgehammer and hacksaw to the new 60GB Xbox 360 to see what changes have been made inside. What he found was ... well, pretty much the same stuff that's been there for a bit. Sure, there's the bigger hard drive, but the only other main difference seems to be the placement of some (allegedly) heat-tastic memory chips.
There may be some other differences for those looking further back into the Xbox 360's past, but it seems like Heck is jumping off from the point where the extra heatsink had been added. If you've got an original Xbox 360 from way back in the day, there may be a few other changes, but all the cool kids have had their RRoD by now and gotten newer Xbox 360 iterations.