According to Kant, is it permitted under any circumstance to treat another rational being as merely a means to an end? Why or why not?

According to Kant, it is not permitted under any circumstance to treat another rational being as merely a means to an end. Kant held the belief that all humans hold value simply through their existence. His thinking implies that other humans shouldn't be used only for one's own personal gain.
One of the key words when looking at this statement, however, is "merely." He does not say that it is wrong to use another person as a means. Kant believed it was OK to use another person as a means (as most people do multiple times per day) as long as it was done so cooperatively and with consent. An example of this would be taking a bus to get somewhere. By taking the bus, you would be using the bus driver as a means to get to your destination (the end). The bus driver, however, would be using you as a means to earn his paycheck (the end). In this case, the use of the other person as a means is done with consent and with understanding that neither person is simply being manipulated or used.
Kant would take issue with situations in which one person uses the other as a means to an end without consent. This could include a situation where one person makes a promise to another with no intention to keep the promise. An example of this would be if person A asks person B to help him move to a new apartment. In return, person A promises to help person B build a new shed. Person A, however, has no intention of actually helping to build person B's shed. In this case, person A is intentionally deceiving person B and merely using them as a means to an end (moving).
http://www.bbc.co.uk/ethics/introduction/endinitself.shtml