Forget any talk (even our own) about the effect Fox's American Idol would have on Lost's audience; Wednesday's episode wasn't just up 15% in terms of viewers, it was the most successful episode this season.
It wasn't that Wednesday's Lost was the most watched episode of the year (In total viewers, it's currently estimated to be below the first two episodes, in fact). But its rating in the all-important 18-49 demographic was the highest that the show has enjoyed since last season, a 5.1 share of the audience. In addition, Idol's audience actually fell for Wednesday's episode - which may make Fox reconsider the show's move into Lost's timeslot.
As to what brought viewers back to ABC's island drama? We're chalking it up to the island being happy to have all of its children back once again... and the power of ABC's promotions department.