A person converts a traditional IRA to a Roth IRA before 12/31/98. He then discovers that his MAGI for 1998 exceeds $100,000 because of unanticipated dividend income or capital gains. Can he convert back to a traditional IRA in 1999 before the due date of his 1998 tax return without a penalty?

Yes. You have until the extended due date of the tax return to do a recharacterization.