Well, many of us have done just that, and called it OctoberFAST ale, because it wasn't lagered. It won't be a true Octoberfest, which is a clean crisp lager, but it can still be pretty close. The key is to use a clean and neutral ale yeast (maybe Nottingham for dry, or WLP001 for liquid) and ferment as cool as possible within the temp range for the yeast. Mine used WLP001, and it's already gone! I fermented mine at 62 degrees, I believe.
Broken Leg Brewery
Giving beer a leg to stand on since 2006