"Nothing could be less cool, at this point, than calling a startup 'cool.com.' A company with a name like that could not have arisen organically. 'Cool.com' smells of a media conglomerate trying to create a web spinoff."
He has a point, yet getting the do.com domain proves they're taking this quite seriously. I guess the two signals cancel and I'm left neutral about the name. :)
Where that really matters, of course, is for startups like stripe.com and simple.com (formerly banksimple). They have justifiably huge value for their expensive domain names: being taken seriously is life-or-death for them.
Although I dislike all these expensive names -- especially "do" -- on other grounds: http://messymatters.com/nominology
Btw, I'm curious how do.com compares to trello.com...