Here's someone who says that immigration contributed moderately to the rise of wage inequality by increasing the supply of low-skilled labor during the 1980's, but that the effect on native worker outcomes was very slight: http://www.jstor.org/stable/2118131?seq=1#page_scan_tab_cont...

EDIT:

"Some countries even rely upon immigrants to keep their social security system up and running, due to their own citizens aging rapidly and not having enough population growth. I believe Germany is one such example."

That's interesting. Maybe this could help with demographic problems here in the U.S. For example amnesty could capture new tax and social security revenues from millions of younger workers. Demand might be lower at a legal minimum wage however.