Is this really true for the general case:
Atmel app note AVR040 http://www.atmel.com/dyn/resources/prod_documents/doc1619.pdf, specifically section 4.9 with special note of 4.9.2.
It doesn't seem to be mentioned in the datasheets I've been looking at... (ATmega328p etc.) though I may have missed it...
I'm working in a high noise environment, and it is a cause for concern...
Cheers