Edit (1/24/11): I had hoped since the hotfix brought Rejuvenation down to 16% base mana it would stay like that, but in the PTR everything is up for grabs! According to today’s PTR update, Rejuvenation is now 20% of base mana.
An early patch 4.0.6 bonus for Resto Druids!
Rejuvenation now consumes 16% of base mana, down from 26%, but still requires 26% of base mana in order to be cast. A patch will be required to address this discrepancy.
– Zarhym (http://us.battle.net/wow/en/blog/1232869)
As you can read elsewhere, the reason it requires 26% base mana to cast, but actually only uses 16% base mana, is due to discrepancies between the server and client. Blizzard’s hotfix applies to the server, but the client (the game running on your computer) still thinks Rejuvenation requires 26% base mana to cast. When you try to cast it, the server will let your client know that you actually only spent 16% base mana.
The client side of things will be updated in the upcoming 4.0.6 patch, but I am very glad they decided to give us the 10% base mana savings early. During a Bastion of Twilight raid last night I certainly felt a lot less strapped for mana then I used to.