Although this happened during the Tenth Docotr's tenure, in the 2007 Children in Need special Time Crash, it was the first time a NuWho and Classic Who Doctor met, as the Fifth and Tenth Doctor TARDIS' merged together for a few brief minutes. In typical fashion, the earlier Doctor was initially none the wiser that he was speaking to his future self (instead he thought that Tenth was merely a fan of his). The Tenth Doctor subsequently revealed that the Fifth Doctor was "his Doctor" which is in fact true - David Tennant's favourite Doctor is Peter Davison, rather fitting since Davison is now his father in law! Talk about keeping it in the family...