I don't want to beat this dead horse any deader, but this still isn't quite right. The Google results are mostly due to Bukkit, as you noted, and that's not applicable in the cases pointed out here. The signs, in my case, are NEW. I can create new ones today, and they will disappear, along with older ones. Finally and most obviously, it doesn't explain why the problem goes away by rolling back Optifine, without rolling back MC itself.
Recreating the signs when you have many, many dozens is not (of course) ideal, and as I said they will just disappear again anyway.
Fortunately, the data is in the level file; the signs reappear upon restart, and then slowly erode away. So at least they aren't permanently lost.
I suspect it's a chunk loading setting issue. I've played with settings and not sure what, though. I will update OptiFine today and see if it replicates, and report back.
UPDATE: 1.4.6 still has the problem. I created new signs, moved away from the chunk, then moved back in and the errors popped up.
Next test: ran vanilla, fresh download of 1.4.6 without Optifine, default texture pack. Problem does not occur.
Loaded 1.4.6 with OptiFine, still using default texture pack and no mods, and the problem occurs.
The problem occurs in any area with signs. Seems to occur more in one area where I have about 15 signs in one place (probably in one chunk), although this may just be where I notice it more often. Eventually, pretty much all (if not all) the signs in the entire world go blank.
Again, this does NOT occur with MC 1.4.6 running without OptiFine.
I'm having a bug where all dropped items look like stone on SMP servers
I'm not sure if this is your mod though.
Mods I'm using:
Edit: seems to happen on some/most servers
great mod, and iv spotted a bug: when you drop an item with 3D items or something on, I think there not patched if ur running an hd texture pack. all other item are patched but if u drop 1 its not patched.
My fps dropped down to 0-9 (Optifine installed) eversince I updated to minecraft 1.3.2 + up. Are there any solution or reason why is it like this? Because, when I update(removing optifine) or Setting my Video settings to their default values, I get 12-17 fps instead o3o