Yes indeed, it does exist. Only problem with it is that it is really basic and in my oppinion has lots of problems. One thing is that in its current form it is windows only. Second thing I noticed was that it redefined probably every internal constant there was in Allegro. There were bunch of other things I didn't think there should be but I won't list them here.
http://www.skorzec.com/allegnet/On the topic I see no problem with having C# bindings for Allegro. Only thing is that it probably won't be officially supported.