I could be wrong, but isn't it referring to this?
http://www.xfamily.org/index.php/Letter_to_Anonymous_YAs_and_SGAs_from_Mama_and_Peter

Hmmm
If that's the letter to which Peter is responding, well...what kind of a jerk is Peter? (Rhetorical question.) His initial criticism of how the letter was developed (through circulation among friends on the internet) makes him look very defensive. Aparently Peter feels threatened by the fact that SGs form an historical cohort with their own world view? I'm reminded of lines from that poem by Kalil Gibran called "Children":

You may give them your love but not your thoughts.
For they have their own thoughts.
You may house their bodies but not their souls,
For their souls dwell in the house of tomorrow, which you cannot visit, not even in your dreams.
You may strive to be like them, but seek not to make them like you.
For life goes not backward nor tarries with yesterday.

No, to my knowledge that is not the letter the "Professionals" GN was in response to.

I don't know where to find a full copy of Daniel's letter, but you might try contacting thepersoniamnow and asking him if he still has a copy of the letter he wrote.

WalkerJ, you are right. The actual letter can be found within that same document ("The Professionals"). No way of knowing if that is the entire and unedited letter.