Will products based on different ZigBee stacks (like the "Zigbee" and "Zigbee Pro" profiles) be interoperable?
The simple answer is yes, as long as they use the same frequency range and bitrate, as there are multiple options for these in the standard. This means physical layer interoperability (i.e. if you wrote the code for one or both, the modules should be able to talk to each other), not application-layer interoperability (i.e. any two random devices with a Zigbee logo on the box will not necessarily have anything meaningful to say to each other :)
This is covered in the ZigBee FAQ:
ZigBee PRO is the primary development choice; therefore, products built with the ZigBee Feature Set can only participate in a ZigBee PRO network as end devices. Due to the wide variety of products able to use ZigBee, interoperability of products is determined by ZigBee standards.
ZigBee and ZigBee PRO are not different stacks: they are feature sets, and the latter is extended compared with the standard version.
From the same page Nick linked:
ZigBee PRO has a number of optimizations designed specifically for larger networks comprised of thousands of devices.
If you also look at this table:
they are cross compatible, but the PRO version is better optimized for some tasks. The price is higher traffic load and (most likely) $.