Hopefully Cathy and Austin won't mind me throwing in my $.02.
Let me start by saying that these are very observant questions for a non-guitar player (which I assume you are). I'm impressed with your attention to detail.
The phenomenon you're describing is known as "string noise," which I would define as a guitar player unintentionally (usually) making noise when his/her left hand transitions between positions on the neck. This problem happens most often when a player is changing chords, especially chords that are being played on different parts of the neck.
Generally, string noise is something which troubles many beginner guitar players and is considered, by most, to be a bad thing. However, I would describe the golden rule of rock-n-roll as: "If it sounds cool, it is cool." Therefore, some players may choose to tastefully incorporate string noise in order to add an element of not-playing-by-the-rules rawness. One of the most prominent examples of intentional string noise that I know of is the beginning of Aerosmith's Chip Away the Stone. Listen at the 0:07 mark, right before the repeat of the initial chord progression.
[ame]http://www.youtube.com/watch?v=TVeR0bN5j7A[/ame]
Now, having said that, what Joe is doing in those instances that you've identified actually is not string noise. I say this because there is no other reason for his left hand to go up the neck in the way that he does, i.e. it is not a natural transition between otherwise existing parts of what he's playing. To say it another way, he is specifically sliding up the neck with the intention of making that "zip" sound.
It certainly is an interesting technique and I have to say I'm not really aware of anybody else doing exactly the same thing anywhere else.
Somewhat relatedly, I posted about the writing of the solo (and that part in particular) over here...
Yes, you are exactly right. The subtle variation the second time through is not there in the original recording and it is there in just about every other version. The proper guitar-playing term to describe what Don and Joe are doing is "bending," i.e. they're bending some of notes. You can very clearly see Don himself doing it in this video:
- http://youtu.be/JLcL4MkpJQ4?t=5m2s (1st time)
- http://youtu.be/JLcL4MkpJQ4?t=5m27s (2nd time)
I once read an interview, I forget where and when, in which Keith Richards talked about how it was important (to him) to record songs before the band had played them live so many times that playing became second nature. He liked to make sure the recording had a bit of an on-the-edge feeling that came from not being overly familiar with the song. This thought makes a lot of sense and it emphasizes the point that many bands probably record songs in a way which allows for some post-recording evolution.
I suspect Don and/or Joe came up with the bending you're referring to while playing live, probably not long after the recording.
I also think these types of variations are pretty common. Another noteworthy example of the same phenomenon is the presence of subtle differences in the way Jimmy Page played Stairway to Heaven live. Specifically, on the recording, the first note is a single note whereas live, he plays two notes at the same time. Additionally, he adds a little slide variation to one of the chords (the fifth one).
[ame]http://www.youtube.com/watch?v=JOemwDVBlqE[/ame]