Encarta has the answer.  That’s why I’ve bookmarked this dictionary.  Not only is its pronunciation guide a gem but its notes on word usage is also priceless.

In U.S. English, toward is the usual form but in British English towards is more common. The same principle applies to afterward/afterwards and to some other adverbs of direction that end in -ward, for example, backward/backwards and outward/outwards. Upward, as in moved upward, and upwards, as in increases upwards of 10 percent, are also standard.

Note that related adjectives of direction always end in -ward, not -wards, as in a backward glance or an upward trend. The adverb forwards is a seldom used variant of forward in U.S. English, and the -wards spelling of it is never used as a standard U.S. English adjective.