Pearltrees, the Paris-based curation and discovery startup, just launched its long-awaited iPad app earlier this week. The company’s service allows users to bookmark interesting websites and arrange them into hierarchically organized tree structures – or “pearls” in the company’s parlance. I’ve been a fan of Pearltrees ever since I first met the team in Paris about two years ago and have been using their service here for my daily “Catching Up” posts. What makes the service stand out from its competitors is the visual appeal of how you collect and organize your “pearls.” The drag-and-drop interface takes the work out of bookmarking, but while the web interface works quite well, one can’t help but feel that the touch interface on the iPad is actually the most natural way to use the service.