I'm an American working in Finland. This Fall, I'll be in Germany from Sept.–Jan. for work. Since it's longer than 3 months, I know that I need to register my local address. Do I also need to do anything at the Ausländeramt, or is my Finnish residence permit sufficient? (I only get a local honorarium, and am still paid my usual salary and health/pension/etc from Finland.)
Answering my own question to say what happened.
Since I was only planning to stay in Germany for marginally more than 90 days, the Ausländeramt just gave me a Grenzübertrittbescheinigung that lets me stay until the end of the term at the institute I'm visiting. (This is less paperwork hassle and certainly cheaper than getting either a work permit or a long-term visa.)
You do need a German residence permit (or a long-stay visa and/or authorization to work, as appropriate). As a U.S. citizen, you can apply for it from within the country but you are not allowed to work without it.
In general, a Finnish residence permit does not make any difference in this scenario. In particular, it does not grant you any right you don't already have as a U.S. citizen (it would exempt you from short-stay visa requirements but U.S. citizens don't need one in the first place).