Thanks again for that explanation. In my case, room is not an issue so I am tempted to solder an electrolytic in the place of the tantalum. The solder pads are huge and I can bend over the cap so it lies flush with the top of the shield. Will look strange, but that doesn't bother me.
Actually I am using this with the Mega and because of the design of the GSM shield as well as the design of some of my other shields, it won't be possible to have the GSM shield on the stack anyway. Thankfully the GSM shield doesn't require many pins itself so I will just mount it off to the side.
Since it's going to be detached anyway, I am tempted to power the GSM shield with a dedicated 5v supply distinct from the Arduino, but with a common ground. The Arduino regulator is limited in the current it can provide but I can easily make a 5v, 2A supply just for the GSM. This should also make the capacitor in question somewhat less critical in terms of total capacitance.
I can't think of any negatives to this approach but if I am missing something I hope someone will tell me.