Reference: June 3, 2007 NY Times Sunday MONEY section, page 4, article entitled Shattering Stereotypes about Immigrant Workers by Daniel Altman.
Although his article is biased in favor of what he calls "undocumented" workers and he quotes suspect sources claiming illegals don't depress US citizens' wages, the statistics he cites are significant. They do shatter the oft-repeated myth that illegal immigrants are doing work "nobody else" wants to do. Altman gives the following statistics which he obtained from the Pew Hispanic Center's estimates for 2005:

Percentage of [illegal] workforce who work in:
19.9 Construction
14.3 Manufacturing
3.9 Agriculture, forestry, fishing, hunting
0.2 Mining
17.2 Leisure/hospitality
12.9 Business/professional services
11.4 Wholesale/retail trade
5.6 Education/health services
3.2 Financial activities
3.0 Transportation and utilities
1.2 Information
7.2 Other services

I did not find a good internet link to the entire story.