According to Roadfly.com, who consulted with GM, BMW and Toyota on this matter:
Short-term use of lower octane gas should not do appreciable damage to most vehicles. However, the engine will work harder to perform at the same standard as it would with it's recommended grade of fuel. Thus, you will get lower MPG, which, essentially, offsets the savings of using cheaper gas. While newer engines probably won't 'knock & ping' as much as the older ones did, motor noise may increase as it struggles with reduced octane.
Using lower octane fuel consistantly over a long period could prematurely wear out the engine.
