There is a difference between the 9/11 Commission's account of awareness of United 175's hijacking and the version used in the recent CBC documentary Secret History of 9/11.

The 9/11 Commission gives the following account of a conversion between New York ATC Centre and the FAA Command Centre in Herndon:
"NEW YORK COMMAND CENTRE: We have several situations going on here. It's escalating big, big time. We need to get the military involved with us...
We're, we're involved with something else; we have other aircraft that may have a similar situation going on here." (p. 22, 9/11 CR)

However, CBC lays it out a little differently:
"NEW YORK COMMAND CENTRE: We have several situations going on here. It's escalating big, big time. We need to get the military involved with us."
"NATIONAL COMMAND CENTER: We're, we're involved with something else; we have other aircraft that may have a similar situation going on here."
Link:
http://www.cbc.ca/documentaries/secrethistory/timeline7.html

The difference is that in the 9/11 CR version, the fourth sentence, "We're, we're involved with something else..." is said by New York Centre, but in the CBC version it's said by the National Command Centre. If the CBC version is true then this means that:
(1) The National Command Centre knew of various hijackings at 9:01, a time the 9/11 Commission says it didn't;
(2) The 9/11 Commission knew this and covered it up by doctoring the transcripts.

Alternatively, CBC just added in the words "NATIONAL COMMAND CENTRE" due to some oversight on its part.

A couple of other points:
Staff Statement 17 also contains the quote (on p. 7), but there is a blank line between the third sentence and the fourth.
Link:
http://www.9-11commission.gov/staff_statements/staff_statement_17.pdf

Another medium supports the CBC version:
http://americanradioworks.publicradio.org/features/911/b3.html