Mike,
I don't think the problem with your guitar was United as much as it is with Seatac Airport. I live in Seattle and that is absolutely the most anal security when it comes to insturments I have ever experienced. I've never been able to carry a guitar on an airplane from that airport. However, no other airport has ever had a problem.
As for my experience, Northwest domestic flights are a great thing to avoid! My over-seas flights with Northwest were about as nice as an over-seas flight will get however.