No, you won't be dry at 10%, if you specced Masochism. SW:D becomes spammable at 25%, which in turn returns a lot of mana.

I wouldn't stress about this. Blizzard have said the focus is on making Healers work for their Mana whereas for DPS it should be pretty much a non issue - and they have the tools to easily change it.

Further decreasing the cooldown for Shadow Fiend in a Shadow Talent would fix the problem succinctly without really doing much for Shadow DPS.
The same would go for giving a buff to Dispersion Mana return.

There are plenty of ways Blizz can buff Shadow mana regen back up if it's an issue.

@Tom: I doubt we'll see another talent affecting shadowfiend's cooldown. They've already moved the big one to T1 so other specs can grab it, and then we have MF crits that can further shave seconds off the cooldown. I just don't see them making another talent to drop the CD, but perhaps they could boost the mana return for spriests in a deep shadow talent.

Otherwise, maybe they could boost an internal part of our replenishment proccing from MB, using the difference we see in Vampiric Embrace. When we hit a target afflicted with Vampiric Touch with MB, we instantly return 3% mana and proc replenishment for the party/raid.

I'm also a little afraid that we will rely too heavily on SW:D for the mana return. Its no big deal when its on a 10 second CD throughout the fight, but we could end up doing massive amounts of damage in execute. How does the self-damage stack up against our embiggened health pools?

I think the mana regen nerf collateral for spriests is kind of similar to the one moonkins are getting. Because spriests can 'heal' via vampiric embrace, Blizz is nerfing your ability to do damage continuously and, indirectly, your 'healing'.

Moonkins also got gigantic nerfs to their ability to do do damage continuously, because Blizz wants to force them to use innervate on themselves, rather than using it on a healer.

So yes, spriests are collateral damage, but it's not entirely because the other two specs in class are healers, it's also because spriests are 'healers' (of a sort) themselves. Spriest nerfs are collateral to Blizz's desire to make healing really hard.

I am currently leveling a shadow priest and mana regen is a big problem (leveling in BGs mostly). On the other hand my main is a 450 alch, so I go through a LOT of mana potions plus I use lots of buffs (mid 40's). Blizz might be trying to restore some of the values of these items; on my 80's I rarely if ever use potions - only elixirs - so this mioght be a little bit of game rebalancing for professions.
Popping a big potion might be required to get you through some fights - panning rather than faceroll. I like the idea, brings some skill backinto the game that seems to be missing.