Just posted on the WoW forums by Bornakk:

As we have suggested, we have become concerned that mana regeneration is currently too powerful, especially for healers. We want players to have to keep an eye on mana. We don’t want you to go out of mana every fight, but running out of mana should be a very real risk for sloppy playing or attempting content that you aren’t yet ready for. When mana regeneration is trivial then certain parts of the game break down – classes that offer Replenishment are devalued, stats that offer mana regeneration are devalued, and spells that are efficient are neglected in preference to spells with high throughput.

Here are a list of changes you are likely to see in 3.1. They will be available to try out on the PTR. Mana regeneration is somewhat technical, so please bear with us.

• Regeneration while not casting (outside of the “five second rule”) will be decreased. We think that (1) the ability to cast heal over time spells and then sit back and (2) benefitting from a clearcasting proc that also gets you out of the five second rule both provide too much mana regeneration, even over short time periods.
• To make this change, we are reducing mana regeneration granted by Spirit across the board. However we are also boosting the effects of talents such as Meditation that increase regeneration while casting. The net result should be that your regeneration while casting will stay about the same, but your not-casting regeneration will be reduced. This change will have little impact on dps casters, since they are basically always casting.
• The specific talents and abilities being boosted are: Arcane Meditation, Improved Spirit Tap, Intensity, Mage Armor, Meditation, Pyromaniac and Spirit Tap. Yes this makes these “mandatory” talents even more mandatory, if such a thing is possible.
• Since paladins rely less on Spirit as a mana-regeneration stat, we have to address them in other ways. We don’t want to change Illumination or Replenishment. However, we are going to increase the healing penalty on Divine Plea from 20% to 50%. Divine Plea was originally intended to help Protection and Retribution paladins stay full on mana. It should be a decision for Holy paladins, not something that is automatically used every cooldown.
• In addition, we are also changing the way Spiritual Attunement works. In situations with a large amount of outgoing raid damage, as well as in PvP, this passive ability was responsible for more mana regeneration than we would like. We want to keep the necessary benefit it grants to tanking Protection paladins, while making it less powerful for Holy paladins in PvP or raid encounters with a lot of group damage.
• We are also taking a close look at clearcasting procs themselves. One likely outcome is to change them to an Innervate-like surge of mana so that the net benefit is the same, but healers won’t shift to out-of-casting regeneration so often.
• We balance around the assumption that even 10-player groups have someone offering Replenishment. To make this even easier on players we are likely to offer this ability to additional classes, as well as make sure that existing sources of Replenishment are more equitable.
• These changes are ultimately being done to bring the different healing classes more in line with each other as well as to give the encounter team more leeway when designing encounters, who can balance with these new mana regeneration numbers in mind. In a world with infinite healer mana, the only way to challenge healers is with increasingly insane amount of raid damage, so that global cooldowns become the limiting factor since mana fails to be. An example is the Eredar Twins in late Sunwell. We weren’t necessarily happy with that model, and this change hopefully allows us to move towards giving healing a more deliberate and thoughtful pace rather than frenetic spam.

Paladin-relevant parts bolded by me.

I’m not running around QQing about this just yet, until I see some reasoned analysis of the changes, or try it out for myself on the PTR. But you should be aware it’s coming, and I’d recommend analysing your use of Divine Plea to see what it’d feel like if your healing was cut in half when you used it; to keep an eye out for less healing-intensive moments to trigger it.