Well I just had a good read through Kael's modding guide for Civ5 and my heart sank. It seems that the game no longer parses XML on it's own, rather the SDK tool called ModBuddy is used to package it and convert it to the SQL that the game actually reads. This means the SDK is required for even the simplest changes, i.e renaming a city or changing the strength of a unit.
As Alan mentioned, ModBuddy (which is the core of the SDK) requires some Microsoft IDE stuff - not something that Aspyr can port to Mac, instead they (or someone) would have to pretty much rewrite or recreate ModBuddy to work independently or with something like Xcode. What's more, ModBuddy requires a Civ5 installation to function, meaning that if we want to run the SDK via VM we need to purchase a copy of Civ5 for Windows anyway.
Basically it's looking extremely unlikely that Aspyr would give Mac modders anything more than WorldBuilder (map creator). Firaxis wanted to make Civ5 easier to mod (which they have, the SDK is awesome) but by using proprietary formats and Windows only IDE's they're killing it on our side. Aspyr did a poor job on Civ4 but Firaxis have gone and made it even harder for them with Civ5.
Having said that, I can't see any reason why the Mac version of Civ5 couldn't run mods made on Windows so long as there are no DLL changes (just like Civ4). DLL support is not in the SDK yet and probably won't be for a while yet so it's still a big unanswered question whether the DLL would be cross compatible with Mac. I strongly suspect that they won't be, which will rule out all the big name mods yet again.
To sum up this could all create a situation where we rely completely on Windows modders to make their mods Mac compatible because the tools simply aren't available to people like myself to adjust them. I hate to sound so negative so early on, but I think we're potentially looking at Civ5 being worse for mods on Mac than Civ4 was.