My very first instinct while taking the poll was to choose that yes, the employer could pass the cost on to the employee, and no, it wasn't illegal! But, as others have pointed out, it seemed like such an unfair practice, especially given the business reason, that I went against my instincts and voted the opposite way.
My reason for thinking it was legal anyway was that I remember a friend who provides legal counsel mentioning that discrimination by employers, for example, can happen - as long as it is not the type of discrimination covered under the protected classes. Unfair practices can - and do - happen, but that doesn't make them illegal.
Melissa's example likening this practice to employees being asked to purchase their uniforms was great, and this is where state laws would come into play. In California, for example, the employer that requires an employee to wear his own uniform is also responsible for providing it, but it does belong to the employer. This means that the employer can also charge the employee a deposit while it is being used, and if it is not returned in good condition, or at all, the amount can be deducted from the employee's paycheck, or the deposit can be legally kept.
My main concern in this scenario would be the business reason given for this practice - allowing employees to be available 24 hours a day, 7 days a week? Wage and hour laws would become a concern, so a more thorough assessment of the make-up of the workforce would have to happen - how would non-exempt employees be affected? How would their work hours be tracked? How many employees are non-exempt versus exempt? What would be the cost of paying overtime if non-exempt employees were involved? Would different guidelines apply to them, such as only using the company devices during regular business hours?
Employee morale is also an important concern - as the email stated, in that department alone the team "worked hard" already - what would the perception of more work pressure do to their productivity?
If you don't provide your employees with a reasonable amount of positive work/life balance - and the tools and company culture to achieve it - is it sensible to expect them to work at their highest rate of productivity? People produce more, and better, when they feel valued. The act of providing cell phones/handheld devices as tools to aid in better business practices, to increase efficiency, would be positive. The act of using them as a way to increase the workload of your staff could only be harmful in the long run.