So far, Ohio is the lone state to meet the new federal standards. Elsewhere, efforts have been hampered by high costs and legal challenges from the nation's 686,000 registered sex offenders. Advocates worry that the delays are putting public safety at risk.
"This means more of the same - that we're losing sex offenders when they cross state lines and disappear," said Erin Runnion, who lobbied for the law after her 5-year-old daughter, Samantha, was kidnapped and killed in 2002.
"It's incredibly frustrating. How many children do we have to lose to repeat sex offenders before we start taking these guys seriously?"
The initial deadline for states to comply was in July. Then the deadline was extended to July 2010, although several states have signaled they may still be unable to meet it. States that do not adopt the mandates risk losing millions of dollars in federal grants.
The law was designed to keep closer tabs on sex offenders, including an estimated 100,000 who are not living where they are supposed to be. It would create a national sex offender registry and toughen penalties for those who fail to register.
The president of the National Center for Missing and Exploited Children said states need more money to adopt the law, and he called on Congress to help.
"We understand that there are challenges in becoming compliant, but the greatest challenge is that states are overwhelmed. And they're going to need resources to address this," Ernie Allen said.
Last year, a federal judge in Nevada declared the law unconstitutional because it would subject offenders to additional penalties after they have served their time. The Ohio Supreme Court heard similar arguments last month from more than 26,000 sex offenders who were convicted before the law was signed.
Critics have also complained that juvenile offenders would appear on registries in some states. And because the law requires offenders to register in person, it could unfairly burden people in rural areas who would have far to travel.
In addition to the legal challenges, states are also struggling with the cost, which could climb into the millions of dollars.
"We have states being very laid back, and states where legislators are pulling out their hair trying to comply," said Alisa Klein of the Association for the Treatment of Sexual Abusers. "And there's lots of states waiting for another state to bust a move and say, 'We're not going to comply.'"
In California, the state's Sex Offender Management Board estimated last year that adopting the requirements would cost at least $38 million.
Congress tried to encourage states to adopt the measure by threatening to take 10 percent of their federal crime-prevention grants if they do not comply. The grants have swelled with stimulus funding but typically range from several hundred thousand dollars to more than $1 million each year, depending on the size of the state.
California stands to lose a few millions dollars a year if it does not comply, state officials said.
"Obviously this funding loss pales in comparison with the cost of complying with the act," said Dana Simas, spokeswoman for the California Department of Justice.
So lawmakers are locked in a dilemma: They must spend millions of dollars to adopt the system or back off a program that is designed to protect the public from some of society's most dangerous criminals.
"There's a number of issues we're trying to work out," said Vermont state Sen. Richard Sears, who leads his state's Senate Judiciary Committee. "We're not necessarily against the law, but we'd like some money to go along with it to help us implement it."
Congressional leaders may support changes to the law. U.S. Sen. Patrick Leahy, a Vermont Democrat who chairs the Senate Judiciary Committee in Washington, says he has been working on ways to help states comply.
"This legislation makes vital progress toward keeping our children safe, but it only works if states can and do comply with it," Leahy said in a statement. "Rather than punish states that are actively trying to comply with this important federal law, we must work together to address obstacles facing state law enforcement agencies."
Advocacy groups say roughly two dozen states have submitted reports on their compliance for the Justice Department to review, although the government would not confirm that number.
Scott Matson, a senior policy adviser with the Justice Department office that monitors sex offenders, said he has seen a recent uptick in applications, although he did not say whether more states are getting close to compliance.
The Justice Department declared in September that Ohio had "substantially implemented" the law's requirements, leading to a fresh round of complaints. The state's public defender office said court appeals alone could cost $10 million.
Gary Reece is a 50-year-old convicted sex offender who is challenging Ohio's efforts to comply with the federal law.
Reece said he was previously allowed to register once a year and that his name would have been removed after 10 years without a serious conviction. The state's new measure, he said, would put his name on the registry for life and require him to register in person four times a year.
"It's a tremendous burden, no doubt about it," Reece said. "Every 90 days you have to take off work and go register - and if you miss once, you're going back to jail."