I don't have many immediate thoughts on this, so I'll put it up and see where it goes, but this past Sunday's (2008-06-29) New York Times Sunday Magazine had a fairly lengthy and very interesting essay on the precipitously low birth rate facing much of Europe (about 1.3 to about 1.5) and the rest of the 'western' World - with the notable exception of the USA (2.1, which is replacement level, during 2007) - and some of the potential problems that that will create over the next several generations. (What is Canada's current birth rate?)
From what I am aware of, throughout most of its history, the USA has also had a below replacement rate birth rate (the post WWII 'baby boom' years being a notable exception), but maintained a steady strong overall growth rate due to immigration. According to that article, in 1984 the US Census Bureau expected the USA's population to be 309M in 2050. It is now, in 2008, 304M and their projection is now about 450M for 2050 (assuming that the USA's current borders are maintained).