Checked the Bundesbank statistics. Apparently, as of 12/08, Germany's net asset position vis-a-vis the rest of the world is roughly 1 trn €, or 40 % of Germany's GDP.
Net income (as in: income on foreign assets minus foreigners income on their assets in Germany) has hovered around 40 bn € per year during 2006-08. That's roughly a 4 % return on the net asset position. Similar to government bond yields during that time.
Not too bad. Except that it sounds unlikely such returns can continue going foward. Will be interesting to look at the same statistics one or two years down the road...
(Btw, I'm not too sure how these numbers are calculated in detail, and can't be bothered to find out.)