The convention (I use this word deliberately) with the words in question is not to capitalize when used as common nouns. The Shorter OED, 5th edition, echoes the Merriam Webster dictionary, i.e., no capitalization for aikido, judo, karate, kendo, etc), so unless there's some kind of linguistic cabal taking place between publishers - and even between various English-speaking cultures - this is the rule to follow.
Yes, dictionaries are technically descriptive in nature, but their use as linguistic/semantic/orthographic reference materials gives them a proscriptive function, too. That's why it's helpful to own two or three; the reputable ones provide alternatives in spelling and pronunciation for entries, where applicable.
As far as "authoritative sources" in English, there certainly isn't any agreement. Not according to any of the books on my shelf, in any event (books by Amdur, Draeger, Lowry, Skoss - all aikido practitioners - as well as those of R.W. Smith, and Karl Friday do not capitalize it. Nor do the Kodansha translations of Ueshiba Morihei. John Stevens' books do, on the other hand. Those that I have of his are/were Weatherhill publications). Still, the overwhelming majority of authors do not capitalize the gendai arts in their writing.
Websites are the same. Few of the fora dedicated to aikido - Aikido Journal, this site - capitalize, although most of the aikido organization sites do. As a side note, many of them, including the Aikikai's English web pages, have forgotten to capitalize "budo." So much for that argument.
Part of the problem is that these words are, in many ways, neologisms that are still taking root, so who knows what the convention will become over time (and will it be the same in England as in the States? And what will the usage be in other cultures that write in roman script?).
Interestingly, many of the foremost writers of the budo have already given this matter some thought, and they came up with the Koryu Books Japanese Stylesheet, which can be requested here:
Most of the aforementioned writers with aikido experience contributed to it, as did Larry Bieri and Stan Pranin. It gives considerable weight to the argument against capitalizing.