[jmacd]

A lot of hackathons, like Node Knockout, are announced in advance. http://nodeknockout.com/

[frankdenbow]

NodeKnockout may be announced in advance but entries need to be done within 48 hours : http://nodekockout.com/rules

[m0th87]

That link errors out for me. That said, I've never seen time constraints actually enforced in any real way at a hackathon, and I suspect there's a good amount of cheating at events with decent-sized rewards. Multiply that a thousand-fold for a hackathon offering $1m.