Note that this has been edited to reflect the changes in Cataclysm / The Shattering, so the stat weighing is not final and a proper raid-balance of stats is not concrete.
Int provides you with spellpower and a larger mana pool, so it is naturally a strong stat.
Haste is also important, as it allows you to cast your spells faster, and to an extent, reduces the GCD (which makes a difference for instant cast spells). You get a good amount of haste passively through talents though (9% through Judgements of the Pure, 3% through Blazing Light, plus flat cast reductions on certain spells).
Crit is always nice to have, and it synnergizes nicely with some talents (Crit Holy Shocks reduce cast time on HL / DL, for example).
Mastery (at 80 and beyond) gives your heals a bonus shield equal to a % of the heal amount on your target. Shield effects have always been quite effective, and this more or less gives you a permanent Valan'yr effect to work with. I can see this being a very popular mastery, however we'll have to see how well it scales further on.
Spirit is now the go-to stat for mana regen as of The Shattering, and will continue to be so throughout Cataclysm. There may be a FEW items / enchants out there that provide MP5, but for the most part you're looking for spirit for your regen needs. It still remains to be seen just how much spirit is going to be needed to sustain throughout hardmode fights (especially with the ability to Judge Seal of Insight AND Divine Plea as decent mana regen abilities), but regardless, Spirit is the new regen stat.
As for leveling, just worry about Int, Crit, and to a lesser degree, Haste. Early on and leveling, a lot of healing will be done with Holy Shock and Word of Glory, which are both instant. Haste wont play a big role until later when you need to utilize your Flash of Light/Holy Light/Divine Light spells a bit more.
To be honest I think it's a bit too early to say right now. On the PTR even in heroics tanks are getting beaten up pretty hard, as they scaled up monster damage output, so the numbers will probably get rebalanced a bit before the patch goes live.
The biggest thing to get your head around right now is the revamped system of every healer having the three base heals - the slow, small efficient heal, the slow large heal, and the fast inefficient heal. They apparently expect healers to use the slow small heal when there is not much else to do, and to balance the other heals with their cooldowns during heavier damage.
Also, right now I'd pay more attention to what Blizzard says about healing than what the experience is like on the PTR. Here are some recent examples:
The reason I say to watch what they say more than what they do is, they've been very clear that if the PTR behavior doesn't match what they have in mind, they'll tweak the numbers until it does. In the last PTR patch most healers just spammed their slow, efficient spell because it was getting the job done... so in the next patch Blizzard nerfed those spells, as they want them to be only a fallback and not strong enough to stand alone. That sort of thing may well happen once or twice more before the patch goes live.
Incidentally, I find that checking the top page of mmo-champion to be the easiest way to keep track of Blizzard's position on things.
Best Answer
The GCD (for instant holy fire) and the cast time of Smite at 0% haste are both 1.5 seconds. Haste speeds up all three (ticks, GCD, Smite) equally up to 50% haste (when it stops affecting GCD). While evaluating less than 50% Haste, we can just use ticks instead of seconds. (GCD = 1.5 ticks, Smite = 1.5 ticks).
According to the formula in this article, extra ticks occur at the following breakpoints.
So, Unglyphed Holy Fire breakpoints are:
And Glyphed Holy Fire breakpoints are: