Here's an interesting nugget from the latest Economist:

For many, perhaps most Americans, the question is not "Should we welcome immigrants?" but "How many?" A moderate influx may be economically helpful and culturally invigorating; a huge one would be disruptive. It is not easy, however, to look at a proposed law and predict how many newcomers it might let in.

Some estimates are extremely high. Robert Rector of the Heritage Foundation, a conservative think-tank, put it at 100m over 20 years if the Senate bill were enacted. His study, released on the same day as Mr Bush's speech, also included a "maximum" estimate of 193m. That figure—equivalent to 60% of the current population—was seized upon by alarmists such as Rush Limbaugh, a talk-radio host, and Dianne Feinstein, a Democratic senator from California.

But cooler heads queried Mr Rector's methodology. Michael Fix of the Migration Policy Institute, a pro-immigration think-tank, said he doubted that the guest-worker programme would expand as fast as Mr Rector assumes, that immigrants would naturalise as quickly, that so few would die or return home, and that so many would bring their parents. Compounded over decades, small changes in assumptions lead to big changes in results. The non-partisan Congressional Budget Office estimates that immigration reform would add a more modest 7.8m people to America's population over ten years.

That population is growing more rapidly than any other rich country's, but this owes more to a healthy birth rate than to immigration. Granted, some of the higher birth rate stems from immigrant fertility. But the melting pot is still bubbling: adult immigrants work, and their children virtually all speak English. A flexible labour market and stingy welfare state ensure that there is no resentful immigrant underclass with time on its hands, as in France.

Avoiding just such a simmering immigrant underclass is one reason conservative George Will supports a path to citizenship:

As the debate about immigration policy boils, augmented border control must not be the entire agenda, lest other thorny problems be ignored....

Conservatives should favor reducing illegality by putting illegal immigrants on a path out of society's crevices and into citizenship by paying fines and back taxes and learning English. Faux conservatives absurdly call this price tag on legal status "amnesty." Actually, it would prevent the emergence of a sullen, simmering subculture of the permanently marginalized, akin to the Arab ghettos in France.

One last thing: those in the White House and on Capitol Hill should read this and then this (on the "political disaster" facing Republicans) before the GOP Jumps the Shark on immigration.