Generally an employer is not required to grant an employee's request to telecommute.
Employers should adopt a systematic approach to telecommuting requests, perhaps by creating a standard request form which can then be used to track any inequities in the decision-making process. It might be wise for an employer to stay out of the issue of why the employee wants to telecommute altogether, to avoid having to make a value judgment about whether one employee's new baby is more important than another employee's plans to complete their masters degree.
An employer may be required to allow telecommuting as a reasonable accommodation for certain employees with a disability.
You can find more information in the Telecommuters section of the HR Library.