One issue that came up recently is the US law treatment at of "revocable divorces" under shari'ah, or Islamic law. Shari'ah provides a type of divorce called a "revocable divorce" (talaq raj'ee) where a husband may divorce his wife, but if they resume marital relations within a three-month period (known as the iddah) thereafter, the pronouncement of divorce is revoked and the husband and wife remain married. This can only be done twice in the life of a marriage; if it happens a third time, the divorce is final and irrevocable.
The divorce may also become irrevocable (and thereby 'finalized') if the husband and wifes do not resume marital relations during the iddah. Under US law, parties must be legally free to marry each other order to apply for immigration benefits such as a fiancé visa (K-1) or spousal visa. A revocable divorce, because it is not final and may yet permit the marriage to continue, will not be recognized by US law as permitting the parties to remarry.
A marriage will be valid for immigration purposes only where any prior marriage of either party has been legally terminated and both individuals are free to contract a new marriage. See Matter of Hann, 18 I&N Dec. 196 (BIA 1982). It was held in Matter of Souza, 14 I&N Dec. 1 (Reg. Comm. 1972) that both the petitioner and beneficiary must be unmarried and free to conclude a valid marriage at the time the fiancé(e) petition is filed.
AAO Dec. (Unpub), Mar 21, 2012
Practically, then, if the only proof of divorce a person has is a decree of revocable divorce, there must be some additional evidence that the parties did not resume marital relations during the iddah, or else any subsequent immigration petition filed by that person will be denied. Not because the parties are still legally married - but because of a failure of proof that the prior marriage was in fact dissolved.
Ignore this at your own peril. Don't find out the hard way, or you'll have a lot of explaining to do to your new spouse.