I think we've always kind of looked over this because Optifine has been one of those mods that most people use, and personally speaking I remember doing some checks for xray only a short while ago and being completely baffled by the fact that I couldn't see through the lava to check the edit I was viewing. I had thought something was wrong with my client, but it had just been the first time in ages that I didn't have Optifine installed. If you're using the mod, it's one of those things that you just take for granted and don't tend to think about, in my experience. I know of people who swim in lava to find diamonds, and it's honestly slipped my mind that you can't do that in vanilla minecraft.
I looked into what setting this is, and it appears to be the Fog option (Video Settings -> Fog) which you can set to off or fast (or additionally 'fancy' if your graphics card supports it - mine does not) that actually makes seeing through lava possible. It's not immediately obvious (unless you're stood in lava at the time) that changing the setting does this, but the difference between off and on is quite clear.
We've allowed this method so far, but not necessarily in any official sense, so you're quite right to bring it up for discussion so that we can settle on something as a group. I think I would be in favour of moving to make this method of locating diamonds into the category of 'not fair gameplay' as it does require you to use an external mod, however popular it might already be. I'd like to keep this discussion going, as I see we've breached into discussing other mods as well (such as Rei's minimap), and i'm keen to hear player feedback on such things in general. Additionally, if anyone has any counterpoints to the original topic please voice them! There's usually two sides to every argument, so I think it would be good to hear both the reasons for and against allowing using Optifine to locate diamonds under lava.