After the mushrooms were spotted we called the tree man and he came and poked and prodded our tree. Of course by the time he showed up the mushrooms had disappeared (apparently squirrels like them). Anyway it turns out that while there are some initial signs of rot where the mushrooms were spotted, it is relatively minor and of no immediate concern. There was however another spot further up that had a big crack in it where he could put his poking apparatus in a good foot. So it was decided that this part had to be removed. This was somewhat of a relief as we weren't quite ready to have the whole tree come down. After they had removed it they showed us the inside of the branch and commented that the rot had obviously been there for quite some time and that they weren't quite sure how it had stayed up (how comforting...). We asked how it was that this hadn't been noticed earlier given that we have the tree inspected and trimmed every few years. We were told that apparently cracks in this type of tree can open and close over the course of the year and so they are much more obvious some times than others, so it is really a question of when people are looking and the exact timing of things will vary depending on temperature and so it won't even be consistent from year to year. The other interesting thing we learned is that Norway maples in particular tend to branch in a V shape and tend to have some bark pushed out of the way at the joints. This tends to create natural weak points that are subject to cracks and rot. Nonetheless they say that despite all this what is left of our tree looks to be in good health and that we shouldn't have to worry for two years at which point we should have it inspected again. For a tree that is probably about 90 years old that's not too bad if all a bit traumatic.