Glyph of Fading - Reduces the mana cost of your Fade spell by 50%.
Well, I'm willing to guess this is a Minor Glyph. It's not that great and Fade isn't something you spam, though you'll be using it a lot more in PvP than PvE for sure. In the Wrath beta, Fade currently costs 13% of your base mana, according to Wowhead. At level 80, Fade will cost you around 470 mana, give or take a couple of points. With this glyph, it will cost 235 mana. If you're using a talented Fade every 24 seconds it will be equivalent to roughly an extra 50 mana per 5 seconds. According to a friend of mine, 48.95 mp5 is the exact amount. I didn't really feel like getting that crunchy about it, so I'll take his word for it. You probably won't use it immediately every cooldown, but eh.
As a minor glyph, that's fairly decent, really. It sounds underwhelming, but assuming it is a minor glyph, it's likely not a terrible choice for a PvP Shadow Priest. It'll come down to whether or not vastly better glyphs are available.