Working as part of the Model S mfg team, I can tell you it is a single unit. Don't take the #s below as gospel, I'm being purposely vague - I'm sure you understand. ;^)
Cells → modules → packs. X# of [Panasonic] cells become a module, Y# of modules become a pack. Packs are bolted to the underside of the body with 30-something bolts.
I wouldn't give a direct correlation of a battery-swap to an engine swap. Compared to an engine, the battery-pack is quite large & unwieldy. Most of the people doing engine swaps at home are swapping V8s, which generally go in from above, with a hoist. So you're only lifting the weight of the engine (300-700lbs). (Here, we're completely ignoring the fact that ppl who spend Mercedes-money on a car are not the type of ppl who do engine swaps in their driveway... :^) ).
If you wanted to swap a battery pack at home, 1st, you'd need to be able to receive something that weighs over 1000lbs., and has a footprint of something like 6ft x 8ft - not realistic in most ppl's homes. Then you'd need to support both the battery pack & the body - separately, of course, since you're removing it - the body weighing in at over 2000lbs (thanks to lots of Aluminum). Then you have to roll one or the other away - now, using three times the footprint of the vehicle. Then do it all again, in reverse.
You really need a vehicle lift to remove the pack. It will be interesting to see the possible infrastructure develop around swappable packs. If you want to mirror the ICE infrastructure, then this is the way to go. But, I think we need to evolve from that allegory. I'm not going to postulate on what that could be - I work in process & internal logistics planning, not power-train engineering - but I joined Tesla to be part of this exciting future.
I just got to see the 1st Alpha proto run.... And, boy, did that thing run! The production vehicle will be as sexy as the red/silver rollers in all the photos. So awesome....
-bZj