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Someone wanted to place the emblem (coat of arms or whatever the correct English term is) of a city to the article about it and was unsure if this is legal...

It seems unlikely to me that the emblem of a city is copyright. Aren't most of them really old, at the least?

It's almost certainly not copyrighted, but they might still be trademarks. That's no impediment to our using them here, as long as we use them in a way that make it clear that the inclusion of the symbol is for documentation purposes only and does not imply any endorsement or association with the city in question.

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Next question: what's the exact date before which works are copyright free in the U.S.?

There's no way to set an exact date; it depends on how long authors live, which set of laws the work was copyrighted under, and other factors.

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PS: I heard something about a lawsuit Eldrigde vs. Ashcroft at the Supreme Court - will Mickey Mouse law finally be abolished?

Lawrence Lessig is arguing the case before the court as we speak. Wish him luck. http://eldred.cc

Even if the Supreme Court rules one way, it is possible for Congress to change their minds. This is somewhat unlikely because (a) the general public doesn't seem to care much and (b) the people who do care, care a lot (and contribute mightily to congressional campaigns).

The /only/ issue before the court in Eldred is whether or not Congress has the constitutional power to retroactively extend copyrights the way they did in the Sonny Bono act. If the court rules that they do not, there's nothing Congress can do to "change their mind"; "Steamboat Willie" will instantly and permanently become part of the public domain, as will many other works.