I actually killed three party members with a group of cloud giants a few weeks ago. There was complaining. There were accusations that I had made a killer encounter.
The party was level 13, on average. There were four cloud giants -- they were wearing chainmail, but were shorter than normal, and didn't have 15' reach. Other than that, they were right out of the book. They weren't even the magic-using cloud giants.
The party's method of engaging the cloud giants was to run in at them.
Bows? No. Elemental damage spells? No. No, they opted to smoke 'em out with Stinking Cloud (gosh, giants don't have Fort saves, do they?) when the giants were chucking rocks, and then everyone with a weapon ran in. Heck, the party sorcerer actually got into the fight when the party bard dropped the sorcerer's magical sickle.
I gave out normal XP -- although in mind it was "Double it because the encounter went so badly, and then halve it because the party acted like blithering idiots."
So while I would certainly consider "Losing all magical abilities" to be an effect that should affect the XP given from an encounter, I think it's also worth noting that we haven't heard all the sides of the story.
Examples:
There were levers on the back wall -- one of the levers re-activated magical abilities.
The room was dark and filled with shadows, and the party rogue could easily have gotten past the giant or gotten in some good sneak attacks with hit-and-run tactics.
The party had received a crystal earlier in the day from a mysterious hermit, along with a riddle -- "When is a magic missile not a magic missile? Break this crystal when you know the answer." A magic missile isn't magical when magic is suppressed. Breaking the crystal in the room would have reactivated magic in the area.
The PCs turned off the magic themselves through their earlier actions -- destroying a magical pool in the area, using an artifact that messes around with magic, etc.
I don't know that any of those situations happened. In fact, it's possible that none of them happened, and it was just the DM making things hard for the fun of it. But we're only getting one side of the story, so let's not rake the DM over the coals, shall we?
-Tacky