I thought the HD was tied to the specific unit, and couldn't be swapped. That was an idea I had though. Maybe I am wrong.
Definitely not tied to the specific unit and very easy to remove and install on any other Xbox 360 you care to use it on. Makes it very easy to take your save gamed files over to a friends house and use them there (not that you can copy them to his/her storage unit that way, you'd still need a memory unit to use for shuttling content if you wanted to do that).
I've done the swap the hard drives trick a few times on the two boxes I have. Basically one box serves as my spare and is available to my wife and daughter to play with. They very, very rarely use the box in the room for them, so when/if mine dies I reclaim the spare box and have one to use while mine is in for repairs. When mine comes back I return theirs and everyone remains happy while I keep gaming on.
Expensive to have the second box, but not so bad if you get the arcade unit and live with it's limitations. If you have children that would like to play more, it's actually a nice investment as they'll have their own box to play that way (though it would have wired controllers, rather than wireless, unless you buy extra controllers).
Note you can use all of the accessories you currently have with another box *but* you have to 'connect' those accessories to the new box (think wireless and think of how you have to press the little 'connect' button on the wireless controller to get it to mate/sync up with the Xbox 360). It's a painless process, but one you have to go through so the box knows that the controllers it is hearing from are ones it should respond to (and so the controllers know they should be talking to that box).