Which country is the largest source of foreign combatants in the Iraq
insurgencies opposing the MNF and the Iraq Transitional Government.

Dear 

Nice to see you again. The most reliable source I could find on the
subject is the Washington Post:
"The Washington Post reports that most foreign insurgents in Iraq come
from Saudi Arabia, with a significant minority from other countries on
Iraq's borders, such as Syria and Kuwait. The Post culled their
figures from lists of dead posted on radical Muslim Web sites."
(SOURCE: "Rice, In Iraq, Urges Patience" CBS News, May 15, 2005,
<http://www.cbsnews.com/stories/2005/05/16/iraq/main695343.shtml>).

The original article could be found at : 
Susan B. Glasser, "'Martyrs' In Iraq Mostly Saudis" Washington Post,
May 15, 2005; Page A01,
<http://www.washingtonpost.com/wp-dyn/content/article/2005/05/14/AR2005051401270.html>

The Jewish magazine "Forward" quotes  Senator Joseph Biden (D,
Delaware), who has been to Iraq and have spoken with intelligence
officials, substantiating this claim: "Saudi nationals fighting
American forces in Iraq have grown in numbers significantly in recent
months, and are now believed to constitute a majority in the swelling
ranks of foreign Islamic insurgents in Iraq ? and the majority of
suicide bombers in Iraq" (SOURCE: Ori Nir, "Saudi Jihadists Play a
Growing Part In Iraq Insurgency, U.S. Generals Say", Forward, June 17,
2005, <http://www.forward.com/articles/3335>).

Saaudi money, according to CNN, is also at least partly funding the
Iraqi insurgency:
Zain Verjee, "Is Saudi money financing Iraqi insurgents?" CNN  October
26, 2004 <http://www.cnn.com/2004/US/10/26/saudi.money.iraq/>

I hope this answers your question. Please contact me if you need any
further clarifications on this answer before you rate it. My search
terms:
insurgents iraq saudi,  "washington post" insurgents iraq , insurgents
iraq combatants countries , insurgents , "combatants in Iraq come",
"insurgent * in Iraq come", "insurgent * in Iraq", "insurgent in
Iraq", "insurgents in Iraq come", insurgents in Iraq come , "most * *
in iraq come" , "most * in iraq come"

Thanks politicalguru.  
Thanks borisshah.
Perhaps a viable theory that conflates borisshah's comment with
politicalguru's research is that (Saudi) Wahibi jihadis travel to
Jordan on "vacation" and then infiltrate into western Iraq where it
abuts Syria and/or Jordan.

Google Maps shows the long border of Iraq with Saudi, Jordan, and Syria.
The Google satellite shows that much of that terrain is desert.

The Problem is that no one really knows how to track these insurgents
so our best intelligence comes from Arabic news sources who tend to
distort the picture a bit too much. But the majority according to the
US Army supposedly comes from Syria which mirrors the diplomatic row
between the two countries and all the clean-up operations being held
at the border right now by the Iraqi and US Armies

Dear Grthumongous, 

Thank you for that rating and the tip. 

I am sorry, but I cannot avoid my own personal opinion here: some
countries have  landed on the State Department list of states
suppporting terrorism for much less; Some countries are still there,
although some of the "terrorist" they've supported were legitimate
government forces fighting against a US-backed guerilla (by the way,
where since then a peace agreement has been signed); etc.