Apparently executives from both Korean companies, meeting in Hawaii this week as part of an annual summit, discussed the proposal both before and after the event.
While Valve runs a lucrative publishing business, being the studio behind series like Half-Life and Team Fortress, it's their Steam publishing platform—which has arisen in recent years to dominate the PC game distribution market—that makes them such an appealing target for rumours like this.
Nexon, the Korean giant behind MMOs like MapleStory, owns a majority stake in NCsoft, which most recently published Guild Wars 2.
This rumour of course only applies to Nexon and NCsoft's rumoured offer to purchase Valve. Whether Valve, a privately-held company, would ever sell is an entirely different matter altogether.
We've contacted Valve for comment, and will update if we hear back.
UPDATE -- Nexon tells Polygon "There is no truth to this rumor".