While checking for any new information on Wikipedia last night, I found this tidbit added to the Eleventh Doctor page:
In an interview with Doctor Who Magazine, Steven Moffat revealed that the Eleventh Doctor had an entirely different costume until close to the start of filming. The original look had a piratical feel which Benjamin Cook described as "a little like something Captain Jack Sparrow wears in the Pirates of the Caribbean movies". However Matt Smith was unhappy with the costume as he felt it reflected how someone else would dress the Doctor, rather than how the Doctor would dress himself. The eventual costume, in particular the bow-tie, was influenced by Patrick Troughton's Second Doctor, after Matt Smith fell in love with the Troughton story The Tomb of the Cybermen.
This confirms my original belief that Matt's outfit was inspired by Troughton, and it also makes me glad they went with that when I'm now aware of the alternative. Piratical? Jack Sparrow? That would have truly been a nightmare. For all the people who've whined and moaned about Matt's costume, it could have been much worse.
