I am not completely sure if this would apply to you, since I am not an EU citizen.
My experience was this: I was required to surrender my US drivers license in order to receive a German one.
I was informed that it would be held by them for a period of 2 years, after which it would be returned to the American authority which issued it. During those two years, I could trade my German to get the original back.
That is, as far as I know how it works on the side of the German bureaucracy. I would try to contact someone at the office in your district (Kreis) or at the "Bürgerbüro" in your local city hall. Since you will have to deal with one of their officers anyway, they will be able to inform you best of the actual requirements.
But what about EU residents?
After examining several pages and actually reading what the relevant section of the German law has to say, it seems that my experience is indicative of how all foreign drivers licenses are handled by the German government (sources follow).
The city of Cologne has their rules for EU drivers licenses posted online(in German). My experience was almost identical. The releveant snippets are:
Bei Aushändigung des deutschen Kartenführerscheines müssen Sie Ihren ausländischen Führerschein abgeben.
Translation: upon recieving your German drivers license, you will be required to surrender your foreign drivers license.
According to Fahrerlaubnisverordnung § 31
Der auf Grund des Absatzes 1 und 2 ausgestellte Führerschein ist nur gegen Abgabe des ausländischen Führerscheins auszuhändigen. Die Fahrerlaubnisbehörde sendet ihn über das Kraftfahrt-Bundesamt an die Stelle zurück, die ihn ausgestellt hat, wenn mit dem betreffenden Staat eine entsprechende Vereinbarung besteht. In den anderen Fällen nimmt sie den Führerschein in Verwahrung. Er darf nur gegen Abgabe des auf seiner Grundlage ausgestellten inländischen Führerscheins wieder ausgehändigt werden. In begründeten Fällen kann die Fahrerlaubnisbehörde davon absehen, den ausländischen Führerschein in Verwahrung zu nehmen oder ihn an die ausländische Stelle zurückzuschicken.
Verwahrte Führerscheine können nach 3 Jahren vernichtet werden.
Which basically says: You must give up your foreign license to recieve a German one. If the German government has a treaty with the issuing authority, they will send it back to them. In other cases, they will retain possession of it. It will only be returned upon returning the German one. In certain special(read: properly documented) cases, they will make an exception to these rules. After 3 years in their posession, they reserve the right to destroy it if it is still in their possession.
So I apparently either remembered the 2 years thing wrong, or was misinformed.