In some frustrating consolation to my freak-out, Summer was acting only a little more skittish than usual, which would make sense since she was obviously blind on that side. Once I caught her and could check her out, the only symptoms were a horror-flick looking white eyeball and some wetness on her cheek (again, makes sense if her eye was irritated it would run). She was generally in good health and spirits. It was me who was really really upset with myself for not noticing that she was in trouble.
I locked Summer up in the barn and made an emergency appointment with the vet. He checked her out at about 2:30, but didn't call me until almost 6PM. Even knowing that Summer wouldn't die from this, I was still a wreck for the afternoon. The vet finally called to tell me that Summer probably suffered a scratch or a poke to the eye (such as a burr or a piece of hay) and that her eye was trying to do the right thing to heal. It was completely covered over in blood vessels, hence the whiteness, but that should heal itself up in about 4-6 weeks. The vet left me some eye goop to apply two times a day for the next week (oh, Summer's going to love that!) to ensure that no further infection sets into the eye.
If everything works out ok, Summer's sight should be restored with only a small scar in the middle. (If something works out wrong, I should know in the next few days if her eye starts to swell or shrink, and I should call him back immediately.)