The real picke in here is that Firaxis is promissing two incompatible things: extensive modding capability and uniqueness of DLC content ( obviously, otherwise why spend, the bucks in it ? ). Those two together don't make sense at all ... think on this, first you say that modding is easy, give extensive tools for that and such, then you say "I have this unique content you can't find anywhere else ... buy it " ... in this context, what stops me of ripping the DLC partly or completely , use it in a mod of my own and release it? If I'm allowed to mod the game proper, why not the DLC contents ? if I can put a moustache in Liz and use it as LH of Normandy (

), why not Hammurabi ? They are both property of firaxis in the same degree ...
Worse, from the code structure that Civ V appears to have ( Exe, Dll , Lua script, XML with only the exe unaccessible for the modders ... pretty much like in Civ IV ), it is virtually impossible to detect DLC content except by manual examination. Not seeing how Firaxis, in this context, could even check every possible modded game except by exclusive locking ( only firaxis recognized modded versions can run, the others are blocked ... something like HoF mod does to avoid cheating ), a thing that would make modding hard as hell ( because you couldn't test development versions and would need that the player would ask firaxis permission to play with every single minor change he would do ( remember, even changing the autosave interval in Civ IV is modding

) ) ...
In short, you can't have open door to modders and protect DLC content with the game structure publicized so far. One has to give ... and IMHO it will be DLC protection. In fact , I'm willing to bet that the 0-day DLC content will be distributed freely with a patch sooner or later.