From what I hear, in Mamet's original play Alec Baldwin's character didn't even exist. But he expressed his interest in the project and the producers wanted him in so Mamet wrote the character just for Baldwin and lo and behold--it became the most famous scene from the movie. And, certainly, one of Baldwin's most famous scenes.

Matt, that's interesting. Because I saw this trivia item on the movie's IMDB page:

"Alec Baldwin, was initially hired to play Blake (a role which wasn't in the original play), but with the agreement that if Al Pacino was unable to play Roma, Baldwin would play him. Early in preproduction it looked as if Pacino was going to be unavailable, so Baldwin began working on Roma, only for Pacino to join the project and Baldwin went back to Blake."

Seems to confirm what you heard.

I remember not being entirely enthused about seeing this movie (salesmen, and depressing), but so highly recommended, and Mamet, and of course I was immediately persuaded. Afterwards sought out 1969 documentary Salesman (Albert and David Maysles, Charlotte Zwerin), probable inspiration for Glengarry and a potent pinhole view into lives of real sales folk, available on Hulu I just discovered.