Although shutting down Napster won't end MP3 trading, I think there will be a definite downtrend.
A large number of Napster users aren't savvy enough to find MP3s elsewhere, and aren't willing to put in the time it takes to find and get them. Napster made it easy, and once it's gone a lot of people will just go back to buying CDs at the mall. Perhaps if this Napster/BMG thing turns out to be OK, some will be willing to pay, but I doubt it. Just my 2 cents.