GamersGlobal managed to pin down Mike O'Brien and Jeff Stratin, two of the ArenaNet founders to find out what's been happening at ArenaNet, GuildWars, GW:EN and their reasons for leaving Blizzard. Here's a snip from the interview:
GG: You’ve already announced that Guild Wars 1 will still be actively supported by its own team when the successor has been released. But there will come the time, when, after all, Guild Wars 1 will be over.
Jeff Strain: Our goal is to make Guild Wars 2 so phenomenally cool that no one will even consider not to play it! If we succeed in that, then we expect the vast majority of players is going to play Guild Wars 2. However, some will be willing to continue GW1. And as long as there are people who play Guild Wars 1, we will keep that game going. We’re not going to shut down servers and kick thousands of people out! There may come a time where we will not keep adding new content to GW1, though.