Well I just tried what you said to see if I got prompted for a apple ID and password. I booted into the Recovery partition and selected install OS X Lion. It then asked for an internet connection so I plugged in an ethernet line. Then it "verified computer eligibility" and began downloading from the internet. It says it will take over 3 hours to download and install Lion. Will this create a more clean install versus the Mac App Store USB stick method?
If you erase Macintosh HD before the install, it is a clean/fresh install the same as doing it from the USB key. Either method will require an AppleID for a reinstall later.
What happens when you start that install is Apple's servers look at your serial number to see if the machine came with Lion/Mt. Lion. If it did, you will get the OS DL free. If it did not, you will be asked for the AppleID used to purchase the OS from the App Store.
It is odd you are not being asked for the AppleID. You normally only get the free install without AppleID if the machine came with Lion, and being a 2010 model, yours would not have.
This is an issue that keeps coming up with selling machines that have been upgraded to Lion/Mt. Lion. The upgrade belongs to your AppleID and technically you can't give it away with the machine. That said, it is not like Apple has the Apple Police out looking for people who have violated the licensing agreement in this way. The only snag is like I mentioned, the new owner will be disappointed if they ever try to reinstall.
To really be totally legal you should fresh install from the DVD that came with your Pro. I assume that was a Snow Leopard installer.
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Can I download the OS X Recovery assistant from Apple's website, save the Lion installer to a USB stick on my macbook air and then use that USB stick to reinstall Lion on my Mac Pro? Would that method be any better so its not tied to my Apple ID? Surely this situation has to have come up before with MP owners. I just want to securely reinstall Lion so the new owner can start it up as it was new, but not have it tied to my Apple ID.
There is no way around having the enter an AppleID at reinstall from recovery. However, if you do that yourself and just quit at the end when the setup wizard starts, it would be like turning on a new machine to the new owner.
The new owner will only ever have an issue if they do a command-r and erase then try to reinstall.