The ppi of the iphone display is already higher than the ipad, so why double the resolution? That one doesn't make much sense to me.
And for two, it should be noted that competing devices released in the past year have higher density displays. Also higher density displays allow for better looking user interface. I would imagine Apple wouldn't want to be outdone in either respect.
In short, I don't think we should draw conclusions neither from Gruber's post, nor from the densities of other devices Apple may be selling.
Though to be honest, I think it's just the constant march of technology, more ppi is better right up to the point where you can't distinguish any improvement.