Thanks for the reply.
There are plenty of game engines with full TMX support, so the overhead can't be so significant that it's not worth doing.
Plus it's something that you could only include if they user chose to. If you need it, you pay the cost.
Another option would be to import the TMX files in the editor and convert them into your internal tile map format.
That way, we could have artists use Tiled to create game levels, then import them into Unity/tk2d and avoid any additional overhead of XML parsing, Base64 or ZIP.
It would need to be easy to refresh the tile map if the TMX was modified (ideally automatically, but on demand would be okay).
Anyway, just some thoughts.