I asked this very same question in the original Holy Priest Theorycrafting a week or so ago. Based on the responses I got, a fellow guildmate and I played around with gear, gems, and enchants on our own spreadsheets and he respeced on PTR. With all of current TBC content clear and with 10-man zones not really being our guild's focus (even with Zul'Aman cleared on PTR), we were more looking to see if it was something that would be a raid min/max question come Sunwell.
Our position was that this person would take on the role of buffer/raid healer. This is based on his talent placement and responsibilities. He is in T6 loot across the board, so I understand our test is very narrow-focused. We didn't change any of his current gems or enchants, though we would probably replace a lot of the
[Sparkling Empyrean Sapphire] with
[Royal Shadowsong Amethyst] and factor in some of the
[Teardrop Crimson Spinel] to counter the loss to +healing from Talents. Overall, the gear set would be more geared to +healing and mp5 rather than Spirit. This is because the scaling benefit received from Spirit in a PS spec, is much less without talents like Spiritual Guidance and Spirit of Redemption.
Results from PTR and Zul'Aman so far have led us to believe that this is a raid viable spec. Is it a maximized healing spec? No. However, it does appear that it can offer a raid maximization perspective. The staying power of a PS spec (even in Spirit gems and enchants) is still there. The healing output, in T6 equivalent loot, isn't so significant a loss and will only improve with adjusting gemming. Granted, if you have a person obsessed about topping healing meters, this spec isn't for him/her. To me, this spec represents the ultimate support role. Overall, in a matter of min/maxing a raid, there could definitely be situations come Sunwell where PS is huge.
I don't believe it's a critical spec to have the moment 2.3 comes out. Nevertheless, I do believe some Priests in each guild, should look at the gear they acquire now and take into consideration if they will spec PS come Sunwell. In that regard, at least you're saving some money and guild resources.