The multiverse concept has shown up in many forms of media, from books, to comic books and TV shows. According to Wikipedia;
The multiverse (or meta-universe, metaverse) is the hypothetical set of multiple possible universes (including the historical universe we consistently experience) that together comprise everything that exists and can exist: the entirety of space, time, matter, and energy as well as the physical laws and constants that describe them. The term was coined in 1895 by the American philosopher and psychologist William James. The various universes within the multiverse are sometimes called parallel universes.
With the server structure that Guild Wars 2 will be built around and Tri-World combat being a part of the lore it suggests that Guild Wars 2 will fully embrace the multiverse concept. Thinking about this gets the gears in my head turning as I imagine the different opportunities this system affords role-players.
On a small scale the multiverse concept can be used to create fun stories and characters such as an asura and their sylvari companion that travel from universe to universe having madcap adventures. Another archetype could be a like the planes-walkers from magic and DnD who are planar transients.
Another fun idea would take a great deal of time and effort and hopefully result in large scale role play. Two players roll characters on different servers who begin prophesying of an eventual conflict of the other server. As time passes demagogues from each server begin to speak out against the other and rally the troops for war. When the two sides eventually clash it will be with the ferocity of rivals rather than simply someone from the same bracket of servers.
There are many more ways to play the multiverse card in role-playing feel free to voice your ideas and opinions in the comments below.