This is my Armory:
My Holy priest is my alt that I geared up to mostly 25-man gear. Not BiS or anything but lucky enough in 25man PuGs that she got stuff. I went in to Ulduar10 with a solid group and had very little problems in terms of mana. Obviously I outgear the place, but even if I were in 10-man gear I don't think I'd be in a bad spot. I ended most fights with a very comfortable level of mana. And as players above me have commented, I've found it's less an issue of healers going OOM so much as certain situations throwing out so much damage at once that you don't quite keep up.
I haven't gotten all the way in (10/14) but thus far at least Holy priests are in a fine place in Ulduar10. I believe Hard modes will force me to trim some of my slack (sloppy overhealing mainly) but such is to be expected.