This isn't an official change in policy, but the word among immigration lawyers is that U.S. Citizenship and Immigration Services (USCIS) is giving greater scrutiny than ever before to prior marriages, in cases where the current marriage underlies the person's request for immigration benefits. This can affect applicants not only applying for a green card through marriage, but also applying for naturalization (U.S. citizenship), if they got their green card through marriage, or even if they got the green card in some other way but appear to be committing bigamy.
The core issue is USCIS's doubt about whether the documentation of the termination of the prior marriage -- most likely a divorce certificate, though death certificates might also receive the same scrutiny -- is the real thing. As you likely know, submitting copies of such documents is a required part of applying for U.S. immigration benefits based on or involving a current marriage. But with such documents coming from countries around the world, it can be difficult for USCIS to assess their credibility and detect the fraudulent ones.
However, improved communications technology is giving USCIS and other immigration authorities new prospects for investigating a document's source and validity. So even if you believe you've done everything right, you may find that your case is delayed, or you are asked for additional documents, if the U.S. immigration authorities start investigating whether your first marriage really ended.
The stakes are high here. If you are seeking a green card based on marriage, and your marriage took place despite your not having received a valid divorce, then your current marriage is invalid and the green card will be denied. Similarly, if you are applying for naturalization, USCIS has a whole new opportunity to check whether your marriage-based green card was valid, or to assess whether you have the good moral character required for naturalization. See Nolo's article "Will You Be Denied U.S. Citizenship Based on Polygamy, Bigamy, or Multiple Marriages?" for more information.