[gripe]
(why the Arduino boards don't use the same crystal for both chips anyway is beyond me - and why use a crappy RESONATOR on the microcontroller when some applications may need some semblance of precision?)
[/gripe]
It seems to me the very first version of the Uno (and maybe mega2560) had crystal resonators and the story was they couldn't pass FCC standards for emissions tests without changing to the ceramic resonator. Not sure that was fact or conjecture, but I'm pretty sure that story was put out there by someone.