This ability (to re-flash 1,2,4,..or,128 bytes)

A warning on this - maybe not so much for program flash as for data flash.

The Atmel byte write is accomplished by the chip itself loading a page of flash into an onboard RAM, updating the RAM, erasing the flash page and then writing the flash page from RAM.

This works beautifully, BUT the max writes is per page, not per byte.

A design I inherited assumed the endurance was per byte and after 10.000+ writes to the same page, some flash chips started failing (as per spec - no blame to the chip manufacturer).

So, if you plan 10.000+ writes to the flash over the products lifetime (3 years for 10 writes/day), make sure you spread it over several pages regardless of whethet it is a byte write or page write chip.

Erik