I have a battery-operated Headroom portable that does a fine, quiet enough job of giving an iPod or my laptop enough voltage to drive a pair of pretty inefficient cans (thinking HD650s, K701s, Etys without an i in the name...), but if I had it to do over again, I'd probably get with with an integrated DAC that would run off of USB as well as analog. If you have headphones that are compatible with the voltage output of and efficient enough for the power of the iPod, I'm unconvinced that a portable amp buys anything more than a whole other gain stage and all of the opportunity for the introduction of noise and distortion that implies. But if it gets the digital to analog process out of the iPod and the MacBook, that might be a net gain, depending, of course, on the quality of the implementation.
Tim