I've read on the forum that we currently have a new world that will generate into 1.8 once stable bukkit for it comes out.
I'm just wondering if we have like an invisible wall there or is it infinite ocean?
I've been travelling around a few times and found several small islands, one of which I have colonised.
But am a bit worried that once the server enters full 1.8 mode, the terrain will generate all around my place and I will spawn my self into the stone coffin.

I'm afraid the only person that can answer this one is Broonie (unless one of the other staff already know the answer)

I think you will find that the 1000 x 1000 border we have now will remain 1.7 gen and once updated to 1.8 everything outside that will be 1.8 gen so you shouldn't need to worry. I believe however that sunken temple chunks are actually placed (without generating anything) in the previous generated land so we might find patches of ocean spawning temple guardians without temples to guard.

apparently, they have something called "retro-gen" in 1.8 which will add the temples in already generated chunks, however they did say something about how old the chunk was and how developed it is as not to destroy anything you build.