This one is quite specify: specify ttf font rendering font-weight:bold incorrectly on iOS mobile safari, open demo site with iphone/ipad with iOS 4.2/4.3 Beta 3
font-weight:bold
don't use the 'bolder' or 'bold' tag. they aren't necessary if you are using a specific weighted webfont.
I had the same problem. It went away when I removed any mention of font-weight.