In the Spark Store there seems to be entries for two different Shield Shields. One for the Core and another one (not for sale yet) for the Photon.
What is the difference between the two? I thought the Photon and Core were pin-compatible.
I just ordered a Core Shield Shield to use with the Cores I currently have, but I also have several Photons on order. I hope I don’t have to have separate Shield Shields for each type of module.
The Core and the Photon are almost pin compatible. They are entirely pin compatible if you don’t use any of the pins that have changed.
Specifically, the Photon has a Wakeup pin and a VBAT pin, and the core doesn’t.
You can read about all the changes here: https://github.com/spark/photon/
As far as I know, even the pins that have got new labeling and additional features, can still be used as before.
The 3V3* (filtered output) seems to have gone and has been replaced with an battery in.
Edit: Not seen @naikrovek has added the same linke already 
For that reason have a look at
Given this I’d guess that a Spark Core Shield Shield would still work with a Photon plugged in, but the extra features might not work, since especially the DAC might require some slight alterations in respect to level shifting and stuff, but this is only a guess, and an official word of @Dave, @zach, or @mohit might be good here 
We're updating all of the shields based on all the things we've learned over the last year. I believe Core shields will work with Photons, and Photon shields will work with Cores; we just were trying to communicate on the store that you could buy a "new, better" shield that would ship with Photons rather than with Cores.
I'm not aware of any compatibility issues with the shields (old or new), but @mohit would know for sure.
The current shield shield has some I2C communication issues and might be fixed in the new version
Thank you. It would be nice to have this kind of detail in the store descriptions.
Since I’m trying to do development work with several Cores, I imagine I would not have waited, but it would have been nice to have known this. I ordered a Spark Shield and it arrived very quickly, along with the 8 Core modules.
I haven’t needed or tried to use it yet.