I'd say that time will be after this World Cup. Surely at least one, maybe both Hayden and Gilchrist will give the one day game away. Gilchrist is still saying he loves playing and won't be retiring. Plus Gilchrist is better at ODIs than Test cricket at present, so I don't think he'd want to switch to tests only.
But at a guess, I'd say Hayden would retire from ODIs after this, especially if Australia wins. (And if he doesn't, he should get a gentle push from the selectors as there's no point in him hanging around for a couple more years, as he won't be a factor in the 2011 World Cup)
Then Watson would step straight into the opening gig. Unless of course he starts hitting like Afridi at #7...