This is a very good question, because the ability to link your purchase to your iTunes account it tied in within the actual purchase from the Mac App Store. I have read, but cannot confirm, that when you purchase a new Mac that has the drive included, they will "credit" this against your iTunes account (or provide a facility to allow you to do this).
New Macs with the stick included are in an odd situation, because they come with the drive, but also come with the Recovery partition included as Firmware (not as a disk partition) that will allow you to re-install online. The online install option required authentication to prove validity, so this would be useless if they did not make the connection somehow. I think actually it's related to your system serial number, they know it was sold with Lion, so they will let you download even without a purchase history or Apple ID connection.
It should be noted that the Ts&Cs for the stick suggest that once you have installed via the retail thumb drive, you cannot then subsequently reinstall or recover using the recovery partition, in fact I believe that it does not create one. Some suggest this is because there is no mechanism to link the thumb drive purchase to your Apple ID.
I think the best solution is to avoid the retail stick, purchase Lion through the MAS, and then burn your own install DVD/USB/HDD installer from the package that you download. This get's it linked to your iTunes account and provides the license to do multiple installs, and provides you with both the means to do a full local restore without internet connectivity, as well as online recovery.
Whether this is technically against the license to install on your partners machine is hard so say, depends, I suppose you could own the machine, but just not use it as the primary user - there is no problem with his/her machine being subsequently used by a different iTunes ID. The spirit if the MAS license is all your home PCs, which I take to mean all the ones in your home, not just the ones which legally belong to you. Unlike purchases from the MAS the OS does not require you to have proof of purchase to install etc.
EDIT: Put it this way in summary, the multiple install license is a feature of the Mac App Store, and not specific to Lion. Purchasing Lion outside of the MAS might not afford you the same rights, certainly it removes the right to do online restores.
Is this on your MacBook Pro? If so, the disks should work both legally and pragmatically. You wouldn't need to use bootcamp though, since bootcamp is designed just for Windows. Nevertheless, it's pretty easy. Here are the basic steps I would follow:
- Insert your install DVD. Shutdown your computer.
- Start your computer pressing the C key to boot from the DVD.
- Once in the installation assistant, go to Disk Utility under Utilities > Disk Utility.
- Select your disk. Select the Partition tab and click on the "+" to add a new one. Call it whatever you please, add at least 10GB of disk space (enough to install Snow Leopard), and make sure it's in Mac OS Extended (Journaled).
- Apply the settings. Go through the installer like you normally would, though when it asks you where to install it, tell it you want the Snow Leopard to be on your new partition.
This should let you boot on Snow Leopard by pressing the opt/alt key, just like when you boot Windows.
Hope it helps! Any other thing you need fear not comment.
Best Answer
There are multiple scenarios in your question. Your final objective is to run Xcode 4.2. Since Xcode is free, you can always try an Apple Store, they may be able to give you a hand with that and install the SL version for you if you charm them enough. That's the simple route, never underestimate an Apple store.
If you want to go ahead and install a Lion, you can connect any external drive (8GB Penn Drive won't cut, too small, too slow), you should rather get a cheap USB2/FW800 (if your hardware supports it) 500GB drive (or even less) and you could use that as the installation drive (make sure it's connected when you boot your Lion's bootable media).
Another alternative (more time consuming but more "secure") is to use an external drive the same (or bigger) size as your current drive, clone your current bootable Snow Leopard into it, make sure it boots (more on this later) and then install Lion in your current drive, as an Upgrade. You can always boot back your old backup drive with SL (and clone it back to Lion if you prefer, including the "hidden" Lion Recovery Partition).
This way you know your original drive stays untouched in the Backup and you can always bring it back. I wouldn't, being so close to ML, I'd just stick with Lion for a month or so, but that's just me.
So how do you boot an external drive or a different bootable partition?
When the computer is starting (and you hear the chime), press and hold ⌥ and wait for things to happen, you should see a list of bootable media (and partitions).
How do you clone your drive(s)?
Using the very reliable and free of charge Carbon Copy Cloner. Rather simple to use.
Remember, after you make a bootable clone, always restart your computer and test the clone making sure it boots and that your data is there. You don't want to find out in a couple of weeks that what you thought was your backup, doesn't work at all.
Last but not least, if you need help installing OS X on an external drive, here's a simple tutorial.
Good luck in your Xcode endeavors!
[thisAnswer release];