It's funny. I broke a spoke yesterday. I took it into my local bike shop, and they asked if I could wait until the next day because they were swamped. I said "sure". After I left, I thought, "I should learn how to replace those myself", especially since I'm dumb enough to ride an occasional century. So I think I'll call them today and ask if they will show me how to make a roadside repair in the event of an emergency. I'll offer to pay them for their time.

I'm with everyone else, though: no need to replace a wheel just because you broke a spoke. Unless you want to use that to justify new wheels....