The mana nerf we got a few patches ago has indeed made mana management something to keep in mind. It has not been a major issue but at least there are reasons to scan the mana bar. There is however one exception that I experienced the other day.
We were, once again, taking down the 10-man version of Emalon the Storm Watcher in Vault of Archavon. I was healing the add-tank and things started of with quite heavy healing already in the pull. That seems to be the normal case before the tanks are in position etc. Things were going smoothly and I kept rolling out my greater heals and renews. The mana was going down so I took a mana pot, used my Shadowfiend, Inner Focus and trinket. Normally this would be more than enough. However it was not, and I got completely dry towards the very last part of the fight. I did announce on Ventrilo that I was running low, and I could see the add-tank desperately looking for panic buttons that was not on cooldown (I can't help myself from smiling a bit when thinking of the poor tank:)). Well the boss went down in time and we got our loot.
Now, we did run without Replenishment in the raid, there were no Innervate nor Mana-tide available, and channeling Hymn of Hope is not on the agenda. Blizzard have announced that they basically expect every raid to contain replenishment, and I think this is where it shows. And I think I can safely claim that it was not a major skill or gear problem (our item-level average exceeds 213 which corresponds to Naxx25), although I think I had to fall back on using Flash Heal more than what should be needed. My overhealing was just below 36000 HP, so for the mana I had there were not much more to to play with.