That will depend on the conversion formula, although I believe that considering the design of current items (for instance let's have a look at
Staff of Endless Winter and
Starshard Edge) , intellect will probably work in some sort of multiplicative way or perhaps items will have coefficients depending on let's say slot type and/or iLvl. That is assuming that Blizzard does not intend to totally "break compatibility" with the current design and rework ALL the "old" items with spellpower, AP, ArP and whatnot (personally I see it as too much needless work). Then there is also the matter of int to mana conversion to consider. You can't just give players endless amounts of int or you risk making mana a non-issue, unless you also adjust the spell costs and maybe damage output accordingly (at which point gg really, since you might need to rework old encounters to take these into considerations too, if you care enough not to break them entirely). Again, I would expect some sort of gimmicky formula to balance things out. All in all, I wouldn't throw the "spreadsheets" to the recycle bin just yet
As stated before, Cataclysm was just announced. We have no idea when exactly it's going to be released (considering the amount of changes I wouldn't hold my breath for the next 6 months to say the least) or even if *all* the recently announced changes will be valid until that time.