No, they'll change each time you break the rock. You're not guaranteed constant items when you break a rock that had one, however. It's completely random.Do the items stay the same each time you respawn them?
Is anyone else surprised by the items your mom buys for you? I've gotten a Muscle Band, a Focus Sash, and a Choice Scarf all from the man at the pokemart.
Edit for Slumdog: You can buy an Abra for 200 coins (at level 15, this is an awesome pokemon for anyone playing through the game), an Ekans in HeartGold or a Sandshrew in SoleSilver for 500 coins, or a dratini for 2100 coins.