Obviously now it would not be but I wonder a few years ago was it ever feasible?
If you're saying hooking it up with a high end ASIC miner, it would be most probably profitable. Raspberry Pi is a machine which uses only 3-5watts. The core is a 700 MHz Low Power ARM1176JZ-F Applications Processor. If you try to mine it, the temperature would definitely go above 100 degrees Celsius with no fan and cooling. You would most probably just damage the CPU.