Here is what J.K. Lasser's Your Income Tax 2010 has to say on the subject:
The value of employer-furnished meals is not taxable if furnished on your employer's business premises for the employer's convenience.
The employer convenience test requires proof that an employer provides the free meals or lodging for a business purpose other than providing extra pay. In the case of meals, the employer convenience test is deemed to be satisfied for all meals provided on employer premises if a qualifying business purpose is shown for more than 50% of the meals. If meals are described in a contract or state statute as extra pay, this does not bar tax-free treatment provided they are also furnished for other substantial, noncompensatory business reasons; for example, you are required to be on call 24 hours a day, or there are inadequate eating facilities near the business premises.
The value of employer-furnished meals is not taxable if furnished on your employer's business premises for the employer's convenience.
The employer convenience test requires proof that an employer provides the free meals or lodging for a business purpose other than providing extra pay. In the case of meals, the employer convenience test is deemed to be satisfied for all meals provided on employer premises if a qualifying business purpose is shown for more than 50% of the meals. If meals are described in a contract or state statute as extra pay, this does not bar tax-free treatment provided they are also furnished for other substantial, noncompensatory business reasons; for example, you are required to be on call 24 hours a day, or there are inadequate eating facilities near the business premises.