Yes, it's just a politically-correct way of saying the legally correct term: "illegal alien". They're "aliens" - people who are citizens of some other country - and they're here illegally.

From this:

The politically correct term undocumented immigrant started with former INS (Immigration and Naturalization Service) Commissioner Leonel J. Castillo during the Carter Administration (1977-81). The word undocumented wormed its way from the widely discredited and now defunct INS to politically-correct media, cheap labor corporations, ethnic vote pandering politicians, and pro-illegal alien 'rights' organizations, many of which are race biased. Even the highest officials including President Bush and former House Minority Leader Richard Gephardt pander to the extent they even call illegal aliens citizens.

An alien is a person who comes from a foreign country. The term illegal alien is broader and more accurate because it includes undocumented aliens and nonimmigrant visa overstayers. An undocumented alien is an individual who has entered the U.S. illegally, without entry documentation. Any alien who violates the terms of his or her admission may be deemed to be out of status. Becoming out of status occurs when a nonimmigrant remains in the United States beyond the expiration date of their visa or when a nonimmigrant engages in employment in the United States for which she is not authorized. Roughly 60% of the illegal alien population are undocumented aliens and about 40% are nonimmigrant visa overstayers. Thus, the term illegal alien, being broader in scope, is the accurate term to use...

A caller to Rush Limbaugh says:

...you'll be glad to know that the term "undocumented immigrant" goes back to the peanut presidency when Jimmy Carter (Watch the Malaise Speech) pointed to a guy named Lionel Castillo as the commissioner of immigration he bragged that his father was an illegal alien, sent out directives that it be called illegal immigrants or illegal worker. Or not illegal, "undocumented workers" or "undocumented immigrants."

More on the distinctions here, and here. See also Euphemisms for "illegal" abound in media coverage of the Minuteman Project. Note that the USCIS doesn't define "undocumented", but they do use "illegal aliens" ( uscis.gov/graphics/glossary3.htm#permanent ). At least, they do that in their glossary. In the article entitled "Illegal Alien Resident Population" (uscis.gov/graphics/shared/aboutus/statistics/illegalalien/), they use "undocumented" in place of the titular phrase.

And, here's a quote from Alberto Gonzalez: Personally I would worry about a policy that permits someone, a local law enforcement official, to use this authority somehow as a club to harass uhh they might be unlawful aliens but otherwise lawful citizens. That would be troubling. That would be troubling to the President.