The attack will primarily involve Marine units -- the 1st and 24th MEU is what I am hearing -- along with Iraqi military forces, largely made up of Kurdish Peshmerga. British forces will deploy to Iskandariya to replace members of the 24th Marine Expeditionary Unit -- approximately 2000 Marines -- freeing them up to participate in the assault. This article in the Army Times and this article from the BBC confirm what I have heard from my other sources.
A contact of mine in Iraq says, "We already knew this was coming. The MEF-1 has been sending out embed notices to reporters that they better be ready to move NOW (within the next seven days) and not to expect to be unembedded for two weeks or more."
**Update: According to the Army Times article, foriegn fighters in Fallujah are only a very small fraction of the total resistance in that city. The representatives of Fallujah said as recently as last week that Zarqawi and his group are not in Fallujah, and that forigners in Fallujah left the city weeks ago. **