Are you sure it is a bug, and not just a difference in the way the vertical spacing is calculated? Apple changed the way Helvetica's height is calculated in iOS 3.x and for a while had a "legacy font metrics" that were available for their apps, which included MobileSafari until they pulled the trigger in iOS 4.x. I have found every browser and operating system and often different versions of each to calculate these metrics differently, even for a specific given font. I talked to some people at WebKit and on Safari about this, and they absolutely do not consider it a bug.