People will continue mining because the block reward includes not only the new coins (50btc right now) but also a small fee component from each transaction. In the future, if Bitcoin has been successful, the value of these fees will be substantial and may even be worth more than 50btc per block.

Sites that make tons of transactions already pay significant amounts in fees to the miners. SatoshiDice.com has paid over 750 BTC over the last 5 months just in fees and all of this goes to the miners.