When a company spans more than one continent, I design the active directory forest based on a geographical model:
Root (Used for adminstration accounts and "universal" groups; but otherwise empty) | +--- americas (includes North America and South America) | +--- emea (includes Europe, Middle East, and Africa) | +--- apac (includes Asia, Australia, and any countries in the Pacific Ocean)
If the number of objects (users, groups, OUs) in one domain exceed 250,000, I typically would break the geographic domain down to country domains such as:
americas (This becomes an "empty" domain and is used to control replication to/from the "forest") | +---us | +---canada | +---brazil
Each of these domains would then follow my design for single-domain designs.