How many House seats will Democrats gain in 2012?

[Small Business Owner of Any Repute]

There will usually be a rebound after a wave election. Waves sweep in some grossly-unqualified and inept politicians and those who otherwise ill fit their districts. People might vote for the Whole Package of the Other Side as a protest of economic conditions -- once, only to find out afterward what they elected. Many Republicans now hold D+4 districts, and they have much convincing to do before November 2012 if they are to be re-elected.

This is almost always not the case, actually. Wave elections are often followed by either status quo elections or further gains for the "wave" party:

1978: Republicans gain 15 in the House in a "strong GOP year."
1980: Republicans gain another 34 in an even stronger wave.

1986: Democrats pick up 5 seats in the House; 8 (!) in the Senate in a great Dem year.
1988: Democrats follow up with another 2 in the House and 2 in the Senate.

1994: Republicans pick up 54 House seats in a historic romp.
1996: Republicans give up 9 in the House, but do go +2 in the Senate.

2002: Republicans gain 8 House seats and 2 Senate seats in a rare midterm performance.
2004: Republicans gain 3 more House seats; 4 more in the Senate.

2006: Democrats pick up 31 seats and take full control of the U.S. Congress.
2008: Democrats add another 21 to their House total while gaining 8 seats in the Senate.

[Small Business Owner of Any Repute]

There will usually be a rebound after a wave election. Waves sweep in some grossly-unqualified and inept politicians and those who otherwise ill fit their districts. People might vote for the Whole Package of the Other Side as a protest of economic conditions -- once, only to find out afterward what they elected. Many Republicans now hold D+4 districts, and they have much convincing to do before November 2012 if they are to be re-elected.

This is almost always not the case, actually. Wave elections are often followed by either status quo elections or further gains for the "wave" party:

1978: Republicans gain 15 in the House in a "strong GOP year."
1980: Republicans gain another 34 in an even stronger wave.

1986: Democrats pick up 5 seats in the House; 8 (!) in the Senate in a great Dem year.
1988: Democrats follow up with another 2 in the House and 2 in the Senate.

1994: Republicans pick up 54 House seats in a historic romp.
1996: Republicans give up 9 in the House, but do go +2 in the Senate.

2002: Republicans gain 8 House seats and 2 Senate seats in a rare midterm performance.
2004: Republicans gain 3 more House seats; 4 more in the Senate.

2006: Democrats pick up 31 seats and take full control of the U.S. Congress.
2008: Democrats add another 21 to their House total while gaining 8 seats in the Senate.

While I agree that people are mostly underestimating the chances of further Republican gains in the House, only a few of your examples support this. Senate gains aren't nearly as relevant because of its staggered elections - a party can gain seats despite losing popularity due to a good map.

Okay, so only 4 of the 5 elections resulted in follow-up gains in the House. If 2012 is anything like the odd year out -- and you can certainly make a strong argument that it will be -- then the GOP will still be holding the House when all is said and done.

I don't think it's guaranteed either, but if the main reason Obama wins is because the GOP candidate for President is lousy (which may be the case!), there's plenty of reason to believe moderate suburban districts will keep their moderate GOP incumbents.

Overall, I think movement will be slight in the D direction. Republicans just picked up so many seats in 2012 (63, more than they got in 1994) that there will probably be some giveback (as happened in 1994). Redistricting complicates things, of course, but overall ...

[Small Business Owner of Any Repute]

there's plenty of reason to believe moderate suburban districts will keep their moderate GOP incumbents.

Like Allen West, Tim Walberg and Joe Walsh?

Well, none of those are moderates, and come 2012, none of the districts they represent will really be "moderate." I already said redistricting complicates things. So no. Not like Allen West, Tim Walberg, and Joe Walsh.