So how do you pass a NULL value to your SPs that use coalesce()?

You raise a point that I don't have a full answer for. If the database column permits NULL values, and the column currently contains data, but now needs to store a NULL, I'm not sure. There's a difference between a null by default and an explicit null. I'll have to think about that one. We ran into that in a survey application where "no response" was an entry - we went back and forth on whether to store a -1 or NULL.