ZigBee Light Link and XBee interoperability
Because the light link specifications haven been released yet, except for members of ZigBee alliance I don't think you will be able to at this moment. Unless you want to become a member of the Zigbee alliance. I have had contact with one of their developers who said that modules that where able to use previous specifications should also be able to use the new light link specifications. 

For now I think this is all I can give you just because there isn't any more information about it. When I receive more information from the developer I shall update my answer.  

Edit: 

I got some more information from one of the developers. About the XBee modules from Digi International this is what information he gave me:


If you go with any of the Pro variants, you will have the ZigBee Pro stack all taken care of in the module.  Yet you will still need some other processing element in the system to (a) implement the ZigBee Light Link Application framework (along with other application functionality you need) and (b) interface with the Pro stack in the module.
If you go with the "programmable variant", that one will give you a secondary processor in the module with 32 KB of Flash and an 8-bit microcontroller.  The Microcontroller would have sufficient power to run the Light Link requirements an the 32 KB should give you just about enough for the light Link framework.  Note that depending on your application requirements (above light link), this secondary mcirocontroller might not have enough left over memory to support that.
If you were planning to have a processor of some kind in your system anyway (for other purposes), it might be more cost efficient to go with one of the PRO variants and place Light Link app framework and your application on this processor.


I hope this makes it even more clear for you. 

Edit:

I got some more information this time not from a developer from Zigbee but from the support of Atmel, you really need to be careful with what processor/controller you pick. Apperently it needs quite some memory power. Atmel only suggests to get: ATmega128RFA1 or the ATmega256RFR2. It is also already possible to get some Zigbee Light Link stacks from Atmel. You can get those over here: http://www.atmel.com/tools/bitcloudprofilesuite-zigbeepropublicprofile.aspx. 

Edit 2:

I got some information that I can't 100% confirm yet, but there is a chance that it will not be able to use the XBee modules just because for a far as I know they are not able to send inter pan frames. These frames are necessary for the commissioning part of Zigbee Light Link. When I get more information that confirms this or not I shall edit again. 

Edit 3: 

The last information got confirmed for the time being the XBee modules wont be able to be used for Zigbee Light Link. The reason for this is that the current firmware doesn't support inter pan commands. It is how ever possible if you would write your own firmware but the source code of the current one is available. They might also implement these inter pan commands in the future, especially since they are needed for light link.