For one Gruber is lampooning WSJ's entrance into the Apple rumors business, so keep in mind he might be mentioning those "specs" in jest.
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.
The iPhone has a higher ppi, but a smaller screen. They may be trying to harmonize the resolution across the devices even though one is physically larger.
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.
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.