That's a bit of an old wives tale. Contrary to popular belief, alcohol doesn't give up that easily.
A study conducted by the US Department of Agricultures Nutrient Data Laboratory calculated the percentage of alcohol remaining in a dish based on various cooking methods. The results are as follows:
Preparation Method Percent of Alcohol Retained
alcohol added to boiling liquid & removed from heat......85%
alcohol flamed.......................................................75%
no heat, stored overnight........................................70%
baked, 25 minutes, alcohol not stirred into mixture.......45%
baked/simmered, alcohol stirred into mixture:
....* 15 minutes.....................................................40%
....* 30 minutes.....................................................35%
....* 1 hour...........................................................25%
....* 1.5 hours.......................................................20%
....* 2 hours..........................................................10%
....* 2.5 hours........................................................5%
Thank you, good information. I stand corrected, so much for mythology or rumor.