Actually, I'm hoping Sakura's element is water. With Naruto's being wind, they'll make plenty of clouds and rain....
(okay, bad naughty joke, there; for those that don't get the reference, it's a common euphemism for sex (or some aspects of sex, depending on certain things) throughout Asia, with seperate origins attributed to Chinese, Korean, and Japanese cultures; one of the three probably came up with it, first, but as much propaganda as the trio spew about what iconic cultural references\foodstuffs\technology\etc. each brought to the world, it's likely impossible to ever figure out which was really the first to come up with something like this)
Seriously, I don't think it'll be water for Naruto. It doesn't feel like he's shown any aptitude, there... though niether has Sakura -- the only one in Konoha who has obvious water aptitute that I've seen is Kakashi. I'm thinking Earth for both of them, and -- while I can't remember exactly what made me think this -- I was wondering if Sakura might have some of the same wood attribute that Yamato has)
As far as I understood elemental affinities, most shinobi only have an affinity for one element... since a kekkei genkai is the ability to combine two to make a new element, I thought that would mean those with a bloodline would be the only ones naturally inclined to two, like Yamato/Tenzou's earth and water. Kakashi demonstrates that his affinity is lightning (the fact that he is not naturally alligned to fire is probably why his implanted sharingan has not developed in the normal way?), and went on to explain that jounin's could usually use two. Since Naruto has already developed his natural affinity, doesn't that mean he could choose whatever the hell he likes when it comes to developing a second element? If that was the case it would make sense for him to choose water to counter Sasuke, but who knows...
When it comes to Sakura's affinity, I really don't know. There was no mention, when Naruto underwent his elemental training, that she had already done something similar - and Shikamaru seemed mildly impressed that he was doing it - so I'm not convinced as to whether she has developed hers or not. Sasuke went through it as a genin, but it took him almost a month, and he was called a genius... so if she hasn't yet, she probably isn't going to until she has a good bit of time. Still, she's had a few years, so I wouldn't be surprised if Kishi just hadn't bothered to mention it... There was that instance with the liquid she used for healing, but I think most medical ninjutsu is related to light and dark manipulation (?), I'm not sure whether Kishi intended it to be seen as water manipulation, though it could be. Also, Tsunade may actually have a lightning affinity - I remember in her fight with Kabuto (long long ago) she used an electric surge to scramble his nervous system, or something...
In short... I really don't know anything until Kishi says it outright
PS. Naruto with a wind-channeling sword would be awesome (SHUT UP SAI)