Dunno if the situation has changed yet ; I was in N-Sulawesi in August, though almost a month before the Bali incident, and i can state that there were no safety concerns for western incl.american divers.
N-Sulawesi is mainly catholic and a trouble free province, but, once again, who could have guessed for Bali?
In doubt, contact Jeremy Barnes at safari tours in Manado (should be something like
www.manadosafaris.com), he's an helpful american guy who runs his own travel agency. You can also check
www.sulawesi-info.com, a useful website for any news around.
Don't miss Lembeh and most of all Bangka dive spots (Batu Mandi and Sahaung are unbelievable), far superior from Manado's.
If you have 4 days to spend overland, don't hesitate going for a 1-2 days trek in northern Tanah Toradja, southern Sulawesi, this is the other place where touring in 100% safe for westerners.
I would not say for Makassar or Bira area, as a bombing occured in a fast food restaurant in december (was it local or tourist aimed?) : unfortunately Makassar Straits waters are too little dived but I've seen there the most impressive coral and nudibranch variety. "as good as Wakatobi but with more fish life" stated the DM I was diving with.