But the use of bitmaps only complicates things when it comes to screen density. If those new devices don't go over 324 PPI, the same bitmaps/buttons can be used.
The problem holding the release of a larger iPhone as more to do with resolution (and thus size) than PPI.
Yeah, resolution is the biggest issue here. A larger device with the the same pixel density would obviously be using a larger resolution screen, which would mean that the icons and buttons would be smaller on a larger iPhone if they were directly ported over. A developer would have to design larger icons to maintain consistency, regardless of pixel density.
For something the size of the iPads, a difference in screen size isn't that big of a deal, hence why the new Mini and Air use the same icons, layout, and resolution. You can still use both comfortably, even if the icons are a little smaller on the Mini. But for something as small and concentrated as the iPhone, it'd be a much bigger deal.
What held Apple up is the lack of auto-layout APIs until iOS6, and the fact that many devs don't even use them now as they were used for many years to deal with a limited number of fixed size screens on iOS.
It was an advantage for developers and helped the Appstore flourish since they didn't have to care about making flexible layouts, but it's become a problem now for many reasons (Apple won't stay at 3.5"/4"/7.9"/9.7" forever)
I think this year will be the ultimate push in that direction and we may even see those devices unveiled at WWDC as an incentive to developpers to make use of those APIs.
The groundwork has been all but laid out by Apple. I just hope they execute on it soon, because I'd love to see more screen sizes come out of Apple.
...which if the rumors of a larger iPhone and iPad are true, I think it's definitely gonna happen.