ArenaNet let drop via their blog today that guesting is real and is coming soon. They hinted at this in the blog post from yesterday, but to see a feature so sorely missed since launch finally have an official date is awesome news indeed. The date that guesting is open to all players is January 28th.
This doesn't come without a hint of not so good news. That date also marks the end of free world transfers. This may end up being a good thing for WvW players and we can finally see server populations start to settle and communities really start to form.
As was discussed in announcements of yore, guesting is limited to PvE, and is not cross data centers (Sorry euro and american friends, the oceans can not be bridged). In addition, guesting is limited to 2 guest servers at at time. This seems like a technological limit on the system more than anything, but it shouldn't pose too much restriction to players who will be guesting often.
It's worth noting again, January 28th is the day free server transfers end, and guesting begins. That means you'll want to find a home and find it quick. It's like a game of server musical chairs. For full details as long as steps to guest, hit the source below.
Source: Guesting is Coming!