Depends on your locale. A collection of facts, much like the phonebook, isn't copyrightable in the US. (It can be in Europe, sometimes.) So, copying the user data and using it in Places might not be actionable.

As for Google Places results being listed higher, if they were search results being artificially boosted, that's one thing; however, showing callouts that provide the information immediately is pretty clearly better from a user perspective, which makes it hard to show harm to the users.

Of course, you can sue over anything, but whether it makes sense to win is another question.