What is the Mac OS 10.7 (Lion) volume encryption standard?
Open Terminal (located in /Applications/Utilities/Terminal.app) and type:

disktutil cs list

Then look for the entry of your standard drive, e.g.

Name:                    Macintosh HD

Look further down to find out the encryption type:

Encryption Type:         AES-XTS



The latter is what I see when using this command in OS X 10.8.2, so you can assume that AES-XTS stands for the AES-XTS 128-bit encryption used in Mountain Lion. 

I expect Lion to use the same encryption model, but I can't verify it because I have no Mac that's running Lion. However, the diskutil command should help you to find the definite answer.