Not for fear of damaging the cellphone, but because it may turn the oven (or other electronic device) on.
http://ca.tech.yahoo.com/blogs/the_working_guy/rss/article/3834
Of course we all know that there is lots of Electromagnetic Interference that can happen as discussed often in the media, books and magazines - IR from TVs and CFLs, radar detectors, cordless phones, baby monitors, garage door openers, neighbour's RF/IR remote, hair dryers, anything wireless, etc.
http://ca.tech.yahoo.com/blogs/the_working_guy/rss/article/3834
...The tenant's ringing cell phone somehow turned on his nearby Maytag oven when it rang...
...preliminary experiments have confirmed that different brands of cell phones can activate multiple models of Maytag ovens...
...The bottom line: Always keep an eye out for oddball effects like this. If you see something strange happening with the electronics in your kitchen -- or any other room in the house -- consider how close your cell phone is when the antics occur,...
Of course we all know that there is lots of Electromagnetic Interference that can happen as discussed often in the media, books and magazines - IR from TVs and CFLs, radar detectors, cordless phones, baby monitors, garage door openers, neighbour's RF/IR remote, hair dryers, anything wireless, etc.