They can't have the same resolution AND PPI. If the regular iPad and the mini had the same resolution, then the mini would have the higher PPI. If they have the same PPI, then the mini will have a lower resolution.
Exactly.
Why do people find it so hard to understand resolution & ppi ?
Despite having the same resolution the ppi of the iPad mini will be greater. 266 ppi (iPad) versus 326 ppi (iPad mini).
That being said if you can't see the pixels in an iPad, then you probably won't notice any difference between the two screens despite the slightly increased ppi on the mini.
The biggest question still to be resolved is around the trade-offs.
What impact on battery life would a more powerful CPU/GPU and higher resolution display have on the 15 Watt battery of the iPad mini.
Remember currently the iPad requires a 42 watt battery to deliver slightly less battery life than the iPad mini's 15 watt due to the retina display and the more powerful 'X' variant chipset (A5X/A6X) which is clearly far more power hungry.
If Apple just grafted one into the other currently, the iPad mini's battery life would end up dramatically reduced to 3-4 hours, never mind heat dispensation issues created with a smaller frame (less surface area for heat dispensation).
That is despite using a more power efficient screen technology (assuming they go for the IGZO technology)
We can assume two things, apple will have to
1) re-engineer the X variant significantly to reduce power consumption to use it in a mini (unlikely and will never be truly efficient).
2) Apple will utilise the more energy efficient and powerful A7 chip across the entire range of iPhone 5S/iPad and iPad Mini.
Assuming they make the A7 a true quad core chip, it may have enough power to drive a 2048x1536 display without need of an X variant.
However as we have heard NO leaks regarding parts or development of the next generation of Apple chips then the chances of any of this happening in the next 8 weeks as these 'Market Anylyst' predictions have stated on the rumor news, we can take them as being nothing more than mud thrown into the air to generate some amount of interest and activity on the stock exchange and not grounded in reality.
So don't expect a march iPad mini retina refresh, (especially as AU Optronics won't be producing the screen till Q3/Q4 2013) and hope that during the summer in preparation for the new iPhone we start hearing some information regarding the A7 chip.