Oh good... I thought you were laying some kind of trap for me (which I guess I'm about to step into).
The argument goes something like this:
If they were called "the twelve" even when there were only eleven, that implies that "apostle" is and office which is passed from person to person (just like the one who held the keys to the house in the OT was an office - starting to see the connection?). If "apostle" IS an office which is passed down, and we already agree that the apostles had authority (Paul spoke of it when choosing not to exercise it), and could speak infallibly (they certainly did when writing scripture)...then where did all the men holding that office go? You want some hints?
I've heard that some translations render that verse "the eleven" to avoid the argument.
After teasing two other posters, I'm going to hit "post" again since this didn't show up.