On the website of the Southern Poverty Law Center, Mark Potok says, in "Who is Jared Lee Loughner?" on January 9, 2011:

At one point, Loughner refers disparagingly to "currency that's not backed by gold or silver." The idea that silver and gold are the only "constitutional" money is widespread in the antigovernment "Patriot" movement that produced so much violence in the 1990s. It's linked to the core Patriot theory that the Federal Reserve is actually a private corporation run for the benefit of unnamed international bankers. So-called Patriots say paper money -- what they refer to with a sneer as "Federal Reserve notes" -- is not lawful.

At another, Loughner makes extraordinarily obscure comments about language and grammar, suggesting that the government engages in "mind control on the people by controlling grammar." That's not the kind of idea that's very common out there, even on the Internet. In fact, I think it's pretty clear that Loughner is taking ideas from Patriot conspiracy theorist David Wynn Miller of Milwaukee. Miller claims that the government uses grammar to "enslave" Americans and offers up his truly weird "Truth-language" as an antidote. For example, he says that if you add colons and hyphens to your name in a certain way, you are no longer taxable. Miller may be mad as a hatter, but he has a real following on the right.

About David Wynn Miller, I found the following:

• Wikipedia entry on "David Wynn Miller"
 
• "'Full Colon Miller'" by Mark Potok, Southern Poverty Law Center, Intelligence Report, Spring 2003, Issue Number:  109

David Wynn Miller is one of the quirkier leaders within the "sovereign citizen" movement, whose ideology has apparently had a role in various other violent incidents too.  It is an outgrowth of the Posse Comitatus movement, which was founded in 1969.  For information about the "sovereign citizen" movement in general, see:

• "Sovereign Citizen Movement" in "Extremism in America" on the website of the Anti-Defamation League.  One of the best overviews I've found so far.
 
• Wikipedia entry on "Posse Comitatus (organization)"
 
• "What is a 'Sovereign Citizen'?", Southern Poverty Law Center, Intelligence Report, Winter 2008, Issue Number: 132.  (Note that this brief description is oversimplified, insofar as many "sovereign citizens" do not use and may be completely unaware of the overtly racist arguments mentioned here.)
 
• "'Sovereign' Citizen Kane" by J.J. MacNab, Southern Poverty Law Center, Intelligence Report, Fall 2010, Issue Number:  139.  (A more in-depth depiction of the "sovereign citizen" movement, including some violent encounters with law enforcement.)
   
• "Message for Students" on the website of the Anti-Defamation League.  See especially "Section 5: The Sovereign Citizen Movement" and "Section 6: The Tax Protest Movement"
 
• "Idiot Legal Arguments: A Casebook for Dealing with Extremist Legal Arguments" by Bernard J. Sussman, ADL, August 29, 1999
 
• "Trusts and the Untrustworthy:  Patriots for Profit" by Mark Pitcavage, ADL, April 7, 1999
 
• "After the latest bloodshed, 'sovereign citizens' -- and the threat of right-wing violence -- rise on media radar" by David Neiwert, Crooks and Liars, July 09, 2010
 
• "Well, maybe a black 'sovereign citizen' will get people's attention" by David Neiwert, Crooks and Liars, August 07, 2010
 
• "Too Weird for The Wire:  How black Baltimore drug dealers are using white supremacist legal theories to confound the Feds" by Kevin Carey, Washington Monthly, May/June/July 2008
 
• "'Savior' in a Strange Land " by Bob Moser, Southern Poverty Law Center, Intelligence Report, Fall 2002, Issue Number:  107 - about a Black supremacist cult with its own variant of "sovereign citizen" ideology

In "Well, maybe a black 'sovereign citizen' will get people's attention," David Neiwert says:

And there's an important thread here: Not only were Jerry and Joe Kane "sovereign citizens," so were Scott Roeder, the assassin of George Tiller, and James Von Brunn, the Holocaust Museum shooter.

A major recent outgrowth of the "sovereign citizen" movement is the so-called "Redemption" movement, about which see:

• Wikipedia entry on "Redemption movement"
 
• "Sovereign citizens -- a 21st Century counter culture" by Andrew Walsh-Huggins, Associated Press, via MSNBC, August 12, 2010.
 
• "Old Wine, New Bottles:  Paper Terrorism, Paper Scams and Paper "Redemption"" by Mark Pitcavage, ADL, November 8, 1999

Some sources, including the above Wikipedia article and AP/MSNBC article, confuse the "Redemption movement" with the entirety of the "sovereign citizen" movement, when in fact the "Redemption movement" is only a relatively recent subset of the "sovereign citizen" movement, dating back only to 1990 or so, according to some of the other sources listed here.

P.S.:

 On Scott Roeder's ties to the "sovereign citizen" movement, I found the following sources:

• "Alleged Killer of Abortion Doctor Has Decades-Long History of Extremism" by David Holthouse, Hatewatch, Southern Poverty Law Center, June 1, 2009
 
• "Sovereign Citizen -- Friend Or Foe?  Some See Anarchism, Others Dangerous Movement" by Johnny Gunn, Nevada Observer, July 1, 2009
 
• Wikipedia entry on the "Assassination of George Tiller"
 
• "Suspected Freeman Arrested With Bomb Fuse" Seattle Times, Wednesday, April 17, 1996 (referenced in "Bill O'Reilly has Dr. George Tiller's blood on his well-stained hands" by David Neiwert, Crooks and Liars, June 01, 2009)

I am not yet able to find reliable confirmation of James Von Brunn's ties to the "sovereign citizen" movement (as claimed by David Neiwert).  But the ADL and SPLC do describe plenty of other violent episodes involving "sovereign citizens."

Further P.S.:  

About Loughner himself, Chip Berlet has called attention to Jared Loughner: Delusuions of Grandeur & the New American Paranoia by Chad Lance, Weird Load: Only in America, Thursday, January 13, 2011.