I am sure I found the answer to this question some time ago but I can't seem to find the answer and I can't remember.
Apologies if this is the wrong place to ask, or if the answer is staring me in the face...
Roughly how many times should I expect to be able to reprogram my Arduino Duemilanove?
Cheers,
LongTom
the first page of the ATmega datasheet says 10000 write cycles for the flash memory
Legend! Thanks very much 
Write/Erase cycles: 10000/Flash memory and 100,000/ EEPROM.
Flash cell Endurance: So it means Completely write and erase entire contents once per day for 27 years, well beyond the life of the hardware.
So dont worry about it.. Its too big number!!
And those are the minimum guaranteed specifications, in reality you can really plan on some multiples of those values.
Lefty