WARNING: If you have not read Chapter 102 of the FMA manga, and don't want to see spoilers, do not follow the cut! Go read 102, and come back when you've recovered!
So Roy's been blinded by his passage through the Gate. Maybe. There are some hints and cues that don't entirely line up, which lead me to think that things aren't quite what they seem. A couple of my fic co-conspirators and I chewed on it, and came up with the following points.
1) Roy didn't open the Gate voluntarily, nor was he looking to take anything from it. He was dragged through it against his will. Ed, Al, and Izumi were all looking to gain something, and lost something as a result. Roy wasn't the one who triggered the transmutation, either. Therefore it's possible he passed through unscathed, and is doing a masterful job of acting. (More on that in a minute.)
2) We didn't get to see Roy's interaction with the Gate...thing. Truth. Whatever you want to call that grinning weirdo, we don't know what, if anything, it said or did to Roy. For all we know he struck a bargain of some sort with it.
3) Pride/Selim doesn't realize Roy's blind until Roy announces he can't see. So Pride didn't see what happened to Roy in the Gate either. Furthermore--anyone else notice Pride covering up his falling-apart face? I'm not sure what to make of that, but it might mean that Pride's not as fully-powered and in control as he'd like us to believe.
4) No one tells Roy that Father's lair isn't pitch black. He decides he's blind pretty much without any confirmation that everyone else can see. He also asks Ed, "Are you with me, FullMetal?" Considering that Ed's been bellowing at Roy for several panels and just grabbed him by the ankle and and face-planted him, and that Envy was destroyed several issues ago, it seems that even if Roy was totally blind he'd have every reason to believe Ed was indeed right there with him. So--is it possible Roy's trying to cue Ed with that question? As in, "Play along and wait for my signal"?
5) Then Roy does something even stranger. He gets up and wanders off across the floor, trips on a hose, and face-plants for the third time in as many minutes. Now, I'll grant that he's had a hard day--very draining physically, a number of serious psychological shocks, probably major head trauma--but all of that would incline me to stay put. Curl up and howl for a while, maybe. Roy has a lot more fortitude than I do, but he's also a logical thinker. He knows for a fact that there are deadly enemies in that room, even if he's actually blind. If he truly can't see, he doesn't know exactly who or what else is there. His hearing may be pretty good, but he's not accustomed to using it to navigate. Nor does he have any reason to get up and walk off--no one's called him, he doesn't seem to be going toward an attack on Selim, if he's blind he doesn't know where an exit might be...the only halfway-plausible reason for that little walk and fall is to cement the idea that he can't see. Which means he's planning something.
6) Roy keeps his eyes averted or half-hidden behind his hands after he announces he can't see. That could be read as a ruse to keep anyone from looking closely
enough to notice if he slips and lets his eyes focus.
7) Roy is a sneaky strategist and he's a master of manipulating people. He'd know that feigning a weakness and encouraging the enemy to think he's won could draw that enemy into exposing a weak spot. Humans are wired to rely on sight first, then sound for "remote detection". If Pride et al can be tricked into believing that Roy's lost that primary sense, they'll quite likely focus on the remaining "sacrifices" and shunt Roy into the "cleanup" category. If he actually does still see perfectly well, that could put him right where he needs to be to finish off the homunculi and save the day.
If Roy's blindness is a ruse, he's giving the performance of his life. It's a &%$# risky trick to try. He's never been through the Gate before, while the others have, which could explain why he lost something this time through and the others didn't lose anything new. I could be reading meaning into dialogue that just doesn't translate neatly and details meant only to heighten the dramatic impact of Roy's loss. He could be blinded for good.
But then, we all expect Al to get his body back, at the very least. Roy could regain his sight, if he's lost it, before the story's through. Or he might end up patched, regaining the use of one eye in exchange for the flesh of the other, making sure he's reminded of the former Fuhrer every time he looks in a mirror. All we can do is speculate, wait for January...and write fanfic. Lots and lots of fanfic.