Like I said, I don't think publishing in print is really tenable due to the global participaton / distribution. I think if we do this, it should be in electronic format, although someone might want to produce a 'print edition' to distribute amongst their own community.
Although themes were touted earlier in this thread, I don't think we should set a theme - I'd rather keep it open, so that the quality / originality of the work remains the most important thing.
Perhaps would-be contributors could submit up to 3 pieces (up to a certain line length of say 100 lines for poetry, or word limit of 1000 for prose), of which only 1 piece could be chosen for the anthology. Although I guess it depends on how many writers submit - if only a small number, then we could look at multiple pieces per writer, although that could lead to fights regarding fairness and favouritism.
We could set up a special thread to house submissions, and set a rough deadline (which could slide depending on response). Then if we wanted to be really democratic, once the deadline was reached, we could set up a poll thread for users to decide which pieces went into the anthology - although we would have to place trust in people not to abuse this system by tipping the voting in their favour, etc.
Anyway, they're just some ideas...