Phillip Zibilich dropped by to let us know that his Minecraft map of the continent of Britannia (or Sosaria, per his labeling of it) will work just fine with the current version of the game and its various new features:

The map has been tested with “Vanilla” Minecraft 1.3 through the current 1.7 Snapshots. Things like Horses and New Flowers/Grasses from 1.6 and 1.7 spawn correctly.

Also tested with various 1.4.x and 1.5.x Feed The Beast mod-pack. New terrain features such as volcanoes and rubber trees will also spawn in.

Good news indeed!