I watched on TV a show where a group of people were discussing
numerology.

One lady was saying how the World Trade Centre attack had a lot of
significance to the number 11.

I can't remember quite what she said, but she gave examples like:

The digits of one of the airline flights added up to 11.
It occured on the 11th day.
The towers look like the number 11.

There were a lot of other examples that she gave, but I can't
remember any of them. I do remember being amazed by the all the examples.

What is the complete "theory" (yes, I do use that term loosely)
regarding the coincidences the number 11 had?

Hi! Thanks for the interesting question.

I have here websites which particularly mentions such instances of the
theory of the number 11 in the Sept 11 attacks. Some of these findings
are the following:

"The date of the attack:   9/11 -  9 + 1 + 1 = 11”
“September 11 has 9 letters and 2 numbers: 9+2=11.”
“The number 911 is the telephone number for emergencies in North
America.”
“After September 11th there are 111 days left to the end of the year.”
“The WTC Twin Towers - standing side by side, look like the number
11.”
“The WTC towers collapsed to a height of 11 stories”

You can read more by clicking on the first two links.

”The Number 11”
http://www.september11news.com/Mysteries2.htm

“Numerology - A Tear For America Memorial website”
http://www.atearforamerica.100megsfree5.com/numerology.htm 

Our next link explains in great detail the numerology theory behind
the number 11 in the Sept 11 attack.

“The Number 11 in relation to the tragic events of 9/11/2001”
http://www.decoz.com/september11_2001.htm 

Search terms used:         
"September 11" numerology
    
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answer, please ask for a clarification if you have a question or if
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Easterangel-ga        
Google Answers Researcher

For a more skeptical view on the relationship between the number 11 and the attacks
http://www.snopes.com/rumors/numbers.htm

The date of the attack: 9/11 - 9 + 1 + 1 = 11
9/11 = 911 Emergency’s Phone number
September 11th is the 254th day of the year: 2 + 5 + 4 = 11
After September 11th, there are 111 days left to the end of the year.
119 is the area code to Iraq/Iran. 1 + 1 + 9 = 11
Twin Towers - standing side by side, looks like the number 11
The first plane to hit the towers was Flight 11
State of New York - The 11th State added to the Union
New York City - 11 Letters
Afghanistan - 11 Letters
The Pentagon - 11 Letters
Ramzi Yousef - 11 Letters (convicted or orchestrating the attack on
the WTC in 1993)
Flight 11 - 92 on board - 9 + 2 = 11
Flight 77 - 65 on board - 6 + 5 = 11
At Ground Zero, Bush put his hand around a fireman with the helmet
number 164.
The ticket number sold to the Arab pilot by Mile High Travel Agency
added up to 11.
 
Also,
On September 11, 2002, the winning New York State lottery numbers were
9 - 1 - 1. The state called it a "coincidence". What makes such a
matter even more weird is that last November 12, 2001, American
Airlines Flight #587 crashed in New York City. That day, the New
Jersey State lottery winning numbers were 5 - 8 - 7!  According to the
Associated Press, many people found these lottery results to be
"chilling and maybe suspicious, too."
http://www.yorknewstimes.com/stories/091302/nat_0913020041.shtml
  
"...This is all part of the Illuminati ritual and ceremony which
relies heavily on Numerology and dates."
http://www.stewartswerdlow.com/archives/sqa/sqa0901b.htm