Publishing state secrets is not illegal in the US. Look it up. Leaking is. But receiving information and publishing, no matter how sensitive that information is, isn't. If it was, the press would be in a terrible place given how many political scandals they've caused through publication of very sensitive, and often classified, information.
As it stands, Wikileaks have broken no US laws, and it will be very difficult to secure a prosecution using the (relic that is the) Espionage Act. It will require: 1) that proof be supplied that Wikileaks is in receipt of a foreign government, and 2) that it conspired with a US governmental representative to leak the material. Neither of these points are in place. 1) Wikileaks has never been accused of being in contact with non-US foreign powers (although Iran has claimed it is backed by the US, ironically). 2) The information Wikileaks uses is always secured anonymously, ensuring Wikileaks is basically just an outlet for information, much like a newspaper.
In short, any claims of potential prosecution against Wikileaks using the US Espionage Act are just that: claims.
It's most likely bluster aimed to intimidate other potential leakers.
