Sorry, just found a newer number (my previous one was for ewnd-of-2003, this one is end-of-2004): resident foreigners in Germany are down to 6.72 million.

_{*Lunatic*, n.
One whose delusions are out of fashion.}

DoDo, I don't happen to have the figures for the DDR offhand (I deliberately excluded them from my Diss to keep the already over large topic manageable - it also includes the Polish settlers, I'm concentrating on expellees from and settlers in Lower Silesia through 1970) but I believe it is about 4 mill. As to unofficial ones - no. Some did move on to other countries but the numbers are small and they were kept track of. I can look them up if you want.  The story of the expellees in the DDR has recently been the topic of various studies, I can give you references if you want. Initially the KPD had the same position on the borders as all other German parties but was quickly forced to change its tune by the Soviets, something which contributed to its loss of any significant popular support in the West even before it was made illegal.

The 1.4 million figure of 'Spaetaussiedler' is not from the USSR but from the East Bloc. Primarily from Poland and to a lesser extent from Romania. In the case of Poland it mainly concerned people who the Polish authorities had categorized as ethnically Polish in Upper Silesia and the former East Prussia. Their actual ethnic identity was a lot more complicated than that.  In addition at the very end of that period plenty of full blown Poles wrangled their way into Germany using their residency in Upper Silesia as their ticket to the West. There is currently a German minority of several hundred thousand in Poland, mostly in the Opole province (Western Upper Silesia). I'm not sure why the discrepancy between my 7.83 mill and your source's 8.1 mill. Mine is drawn from BMV figures.  You should also note that while those Vertriebene who fled in the closing months of the war ended up wherever they found refuge, the expellees proper were deposited according to the rules set by the occupying powers (minus the French who refused to accept any).  In any case in the Western Zones the main area for expellees from what had become Poland was Schleswig Holstein and Lower Saxony (your province). Sudeten Germans gravitated to Bavaria.

In the early fifties the federal government strongly encouraged a better distribution of refugees and that combined with the pull of the Wirtschaftswunder created a large expellee population elsewhere, particularly NRW. They integrated quite quickly but remained at a socio-economic disadvantage to the 'natives' (einheimische) for quite some time and that disadvantage lived on in the next generation.

In political terms the expellees were and explosive group, subject of fears that they would turn radical left, or more likely radical right. When they were allowed to organize themselves politically in the early fifties they formed their own party the BHE which initially did quite well. It faded  partly quickly because it accomplished some of its goals in the form of a massive compensation program, and because of the CDU's skill in gobbling up/destroying all other right wing parties. The BHE's leaders were a pretty nasty bunch,led by the long time Minister for Expellees, old guard Nazi, and war criminal Theodor Oberlaender (who was actually quite a competent minister).  

Germany's policy towards them was contradictory, on the one hand seeking to integrate them socio-economically, preventing them from settling as coherent groups made up of former neighbours, and on the other hand pumping huge amounts of money into preserving their identity as Silesians, East Prussians, etc. so they would be ready to go back when the 1937 borders were restored.

In political terms they then were spread out across the spectrum until they started to turn sharply towards the CDU/CSU in the mid sixties as the SPD began to gingerly edge away from the hardline national consensus on the border issue, culminating in the de facto (but not de jure) recognition of the Oder-Neisse line by Brandt in 1970. They then became a key part of the CDU's base leading to the kind of absurdities like Kohl's reluctance to formally recognize the border in 1990.
Anyways, I think I ran on a bit too much, as I sometimes do on this topic.