The majority opinion (Justices Stevens, Kennedy, Ginsburg, Breyer, and Sotomayor) places on criminal defense counsel a duty to not only refrain from giving misinformation, but to actively provide a warning that is accurate about the possible immigration consequences of a guilty plea (or conviction). The majority held that
"counsel must advise her client regarding the risk of deportation...
We too have previously recognized that "preserving the client's right to remain in the United States may be more important to the client than any potential jail sentence."
In reaching the decision, the majority found that informed consideration of possible deportation can be helpful to both prosecution and defense, as it will give leverage to the prosecution to convince a defendant to avoid the risk of deportation by pleading guilty to non-deportable crimes, and aid the defense counsel by giving them the opportunity to provide a real benefit to their clients if they can come up with a deal that avoids deportation.
The concurring opinion (Justices Alito and Roberts) enumerates some of the very real difficulties counsel will have in living up to such obligation. It also notes how this is a "dramatic departure from precedent" and cites a Cornell Law Review article by Chin and Holmes that found that "virtually all jurisdictions--including eleven federal circuits, more than thirty states, and the District of Columbia--hold that defense counsel need not discuss with their clients the collateral consequences of a conviction, including deportation."
The CNMI Supreme Court was one such court that had considered the issue in the case CNMI v. Shaunglan Chen and ruled against the alien defendant who sought to withdraw her guilty plea for ineffective assistance of counsel when counsel had not provided information about the deportation consequences of her plea agreement.
Obviously, this case will mean some changes by criminal defense counsel are necessary when representing alien defendants. Because the CNMI is under US immigration now, (although in the transition stage), the consequences for aliens pleading guilty or being found guilty of crimes here will be the same as for aliens in the rest of the US. No doubt there will be useful resources (like books, magazine articles, and online articles) to aid criminal lawyers with their duty.
Keeping up-to-date will be the real challenge.
And just to be complete, there was a dissent (Justices Scalia and Thomas).