This is an article I read a few weeks ago, by the lovely/awesome Justine Musk (@justinemusk on twitter). In this new writing world, where authors can be hyper-visible to their readers (and more frighteningly, other industry figures) your online voice ties into your 'brand'. Getting this wrong by trying to present yourself unnaturally could break you. The article is worth checking out.
(And by that I mean, 'click on the damn link and read it already, chumps & chumpettes!')
Tomorrow I'll post a link to an article that discusses character voice, for a slightly different perspective; consider that in both cases you, the writer, are the source of this voice.