The way I see it is it's a nice break from the routine. It's only a problem when it becomes routine. But if it's a 1 time help and they want to pay you developer pay to do admin work, who cares?? You win in that one and if any firm is smart, they won't keep doing that because they could hire an admin assistant for a lot cheaper. Again, if it's a rare occurrence, I say crap happens and get over it. If it's all the time, that's when it's time to bring it up that it's not what you're there for.
When I've had to do boring stuff in the past, it's actually helped my development because now I can write a program or script to help do that boring stuff or it gives me insights into how the company could benefit from a program. I would not have seen these facts without doing the work myself.
However, I suppose if you were truly happy there you wouldn't have Even posted this!