A lot of wanting in the comments in this thread. People really want to believe that one's productivity drops off a cliff after 40 hrs. Whenever I read these types of threads, which so often bubble up to the top of HN, I wonder why people are so energized in trying to convince _others_ that they ought to work less (actually, I don't).
In my personal experience at places I've worked, there has been a strong negative correlation between conference attendance and overall quality and productivity of the staff. Conferences offer such an easy out for people who want to waste a week while pretending to work and taking a quasi-vacation on the company dime that if you're careless in permitting conference attendance, it can easily turn into a free-for-all.
At one small conference I attended, most companies were represented by one or two people, maybe a CXX and a bizdev guy. Our company had a couple dozen people there. The attendee list was published, and it was kind of embarrassing when they acknowledged our presence during the intro remarks because it basically told everyone what a joke of a company we were. The only reason my team was there was so that my boss could add more make-work to put in his quarterly report while earning points from his team for being that "good boss" that supports employee development. It was such a mismanaged mess at that company that they even paid for a team of contractors to take a paid week off to attend a conference, mostly so that they could pad their resumes, since none of that knowledge was actually applicable to the project they were on.
At my next job, people were too busy trying to hit their goals to pitch their boss on conferences in Vegas.
Conference attendance can provide benefits to employers under the right circumstances, but if I had my own company and an employee initiated a request to attend a conference, I'd insist that he have some skin in the game--he'd either have to take vacation days, or if it counted as work days, he'd have to pay for a significant part of the travel/registration costs himself. If he's not willing, then it probably wasn't worth much to him anyway.