As it stands, access to Hulu is restricted in most countries outside the US for licensing reasons. That hasn't stopped overseas users from accessing the site through US-based VPN (sneaky, sneaky), but it appears that Hulu has caught on to this workaround.
As reported by TorrentFreak, Hulu appears to be blocking VPN users from being able to access the site. Although Hulu isn't blocking VPNs directly--that is, it isn't looking for signs that you're on a VPN and blocking you accordingly, it's blocking IP addresses associated with popular IP services.
A VPN block doesn't just impact international users, though; it also means that privacy-minded US residents who surf the Web through a VPN are also out of luck.
But as Engadget points out, users of smaller VPN services and dedicated VPNs--such as an employer-provided VPN--may still be able to access Hulu without any problems, so this may be more of a deterrent than a hard and fast block.
Still, it seems awfully hostile to users--many of whom may not have a deep understanding of the nuances of copyright law and licensing--to block content for users in other nations. It isn't all Hulu's fault--content providers play a key role in these sorts of licensing issues--but blocking users will do little to stop piracy, and will instead likely drive users to torrents or other illegitimate video feeds.