I use Environne brand fruit and veg wash, available at Whole Foods stores, It seems to do a good job getting off that grayish stuff that seems to be on grapes especially. I wash everything except bananas.
The recent awful outbreak of listeria due to cantaloupes (13 confirmed dead, last I heard at the end of last week; check the CDC web site for details) has led to warnings to wash melons thoroughly because when you cut through the rind, your knife carries any nasties that are on the rind right onto the inside flesh that you will eat. A salmonella outbreak in our area about 20 years ago was attributed to the same thing -- people eating unwashed melons that they cut, and the knives took the salmonella right onto the flesh.
I rarely eat melons for those reasons, but if I do, I scrub them with soap and water, and I know other folks who do the same. Any melon has been sitting on the ground in a field, fertilized with who knows what, walked over by who knows what animals or birds -- why wouldn't you wash it very thoroughly, including soap? I don't trust any "rinse" that the farmers or distributors claim they give fruits and veg before I buy them.