Regardless of where you stand on the issue, the ABA Journal reports this morning that the definition has, in fact, changed. It reports that Merriam-Webster's Collegiate Dictionary has included a secondary definition of "marriage" to mean "the state of being united to a person of the same sex in a relationship like that of a traditional marriage." This was also reported in the Adjunct Professor Blog. Apparently, the definition change actually took place in 2003, but giving the change more academic weight is the fact that the Oxford English Dictionary has also included a secondary definition of marriage in draft versions of its latest edition.
Perhaps, this news is essentially a semantic debate as realistically many states have moved to ban gay marriage or to otherwise define the term as a union between one man and one woman. Surprisingly, California was the most recent state to ban gay marriage with the passage of Prop 8. Nevertheless, worldwide, gay marriage/civil unions have gained much more acceptance as this older BBC article discusses.
In the end, whether you support changing the definition of marriage or you do not, the fact is the traditional concept of the American family is changing. No longer is it simply Ward, June, Wally & the Beaver. Regardless of the dictionary's definition, our laws will have to continue to evolve to handle the changing social landscape.