It's kind of hard to charge someone with espionage when they are running another country. Unless the USA is also planning on snapping up Mikhail Gorbachev for espionage the next time he's in town, I think it's a brilliant play.
As a member of parliament, he would only have immunity in Australia from police investigation and arrest, but not abroad.
However, arresting a member of parliament of another country is something that you would usually try to avoid, especially if that person was democratically elected.
Funny thing if he was elected is that you probably could read all the iternal Senate papers on Wikileaks every day ;)
Try telling that to Craig Thomson, MP.
IANAL, but Parliamentary Privilege only buys you some freedom of speech (if you play your cards right), and some protection from civil arrests (i.e. being told to go to court for a civil matter within 5 days of parliament sitting). Note the civil bit - anything the police are investigating will not be civil. "Civil disobedience" can still be criminal, thus not a civil matter.
Leaking Top Secret information will certainly push the envelope on freedom of speech, and nothing will protect him from any prior criminal charges.
While many countries (especially Middle-eastern countries) abuse diplomatic immunity with regards to smaller crimes (and especially parking tickets), most countries choose not to exercise the immunity in the case of felonies (except where the death penalty is on the table). Alternatively, the country will exercise the immunity but will then honor an extradition request.
IOW, unless Assange gets himself appointed as a diplomat of Australia, he's done nothing to improve his situation in the US.
"Having Julian Assange elected to the Senate would give him some advantages, at the very least the embarrassment of the Australian government having to communicate to whatever authority in Sweden - or the USA as the case may be - about 'Senator Assange' or, if ensconced in Parliament, the embarrassment for Attorney General Nicola Roxon if ASIO was found to have tapped Julian Assange's parliamentary phone and internet connection."
Then it goes on about starting a wikileaks party and actually archiving some political change.
The wlcentral article is clearly written by someone who does not understand Parliamentary government (probably a fellow American). Members of parliament are selected by the parties (rather than being elected directly), but the parties must meet a minimum vote threshold to receive any seats.
The senate is a bit different, where you have the option to select either the specific senators you want, or to delegate your votes to a party to allocate.
While Assange will never be part of the government, it puts considerable pressure on the Australian government to do so if he is elected. Letting the opposition languish in prison is a major faux pas if you claim to be democratic.
Note also: you have to be accepted by your host country to be immune from their laws. Diplomats are not a class of people above the law. The US isn't compelled to recognize immunity for anyone until it (for instance) admits them on their diplomatic passport.
It's hard to imagine any circumstance in which diplomatic immunity in the US could come to play for Julian Assange, no matter how wacky Australian politics ever get.
I'm quite certain the US wants to keep its diplomatic immunity when dealing with North Korea, Iran, etc. I'm pretty sure they don't want any country having the option to arrest every diplomat and visiting state official at will.
It's a very dangerous precedent to set. I'm sure China has a list of diplomats, or staff under diplomatic immunity that they would be willing to put on trial for espionage.