3240 is a whole lot of Haste Rating. It should be impossible to get until the second raiding tier and then only available if the heroic gear from that tier has enough pieces with haste rating to put one in every slot. In the third raid tier with heroic gear I expect 3240 should be doable, assuming you prioritize Haste. By the final tier of heroic gear it shouldn't be that hard at all.
2438 is a lot easier to hit. It will still be impossible in blues, and, I would think, even in first tier non-heroic raiding gear. But it will be possible pretty soon as you start to pick up heroic gear or get to the second tier of raiding in Cataclysm.
The significance of these numbers is this: 3240 is what a raid buffed discipline priest requires to hit 50% haste with Borrowed Time up. 2438 is what a raid buffed holy priest required to hit 25% haste (obviously borrowed time is not possible).
That means that those numbers are what a discipline priest needs to get 6 ticks from renew and what a holy priest needs to get 5.
With haste adding extra ticks to the end of HoTs rather than shortening their duration, Haste becomes one of the primary factors that determines how good your HoTs are. Of course Haste plays it's usual role of making their cast time faster, but it also boosts the total amount of they heal and by extension their mana efficiency.
Because of Borrowed Time, haste also completely distorts the relationship that the two healing priest specs have with the priest HoT.
In blues, it will be easy for Discipline Priests to get 5 ticks from their Renews. Borrowed Time virtually guarantees 25% haste in any reasonable gear. Holy priests, on the other hand, will have to settle for 4 ticks. Even though they will likely spend 4 talent points to prop up their Renew, it will ultimately be less powerful than the Discipline priest renew, since Borrowed Time is a more powerful Renew boosting talent than Improved Renew and Divine Touch combined. Somewhere around the second or third tier of raid gear, Holy priests will once again be the ones with the more powerful renews. But then in the final tier of raid gear there is a chance that Discipline priests will acquire a 6th tick (haste is by far the best rating for them anyway) making their Renew once again better than the holy priest.
Now the discipline priest Renew is only powerful under certain circumstances, and a Renew cast on the fly will be weaker than the holy priest. But the only real restriction is having to cast Shield first. Given the current power and cost of Shield, the fact that Shield never contributes to overheal and that it plays well with HoTs, this isn't much of a restriction at all; especially not when you consider that the Discipline priest will be able to cast 5 Borrowed Time Renews for every Shield. Obviously mana is going to be the limiting factor in the number of Renews that can be maintained, not the restriction of casting shields.
The power of Borrowed Time hasted renews is making me reconsider my build to see whether I can fit Improved Renew in there. Talent points are very tight, and I would have to give up something substantial to do it, but it's a real option. If I ever get to 50% haste with Borrowed Time then I will almost certainly take that talent, since that will push the mana efficiency of Renew above that of Penance.
At some point I think Blizzard may take a look at the bizarre math that comes from their decision to have haste add ticks to HoTs and DoTs and decide to reverse it. I would be willing to put down a considerable bet, however, that we won't see that happen until the next expansion at earliest.