The McBans Website offers a warning urging Minecraft players "to change their Minecraft.net passwords ASAP" (thanks Ant). They explain that they have discovered a list of approximately 1,000 Minecraft account names and passwords were made freely available on the internet," expressing suspicions that these were collected "through the use of hacked clients (that pretend to only give you extra features like flying or seeing through blocks... or websites made to look like the official Minecraft site." Here's more:

Given this information, we feel we need to stress that people make sure to only use Minecraft client mods that they absolutely trust and are verified by the community. While this issue brings to the light the malicious opportunities client modders have, we want to stress that many client mods found within the community are not malicious. We believe people should be healthily skeptic but not overly paranoid as to whether or not a client mod is stealing your information. Unless you're using a sketchy client that allows you to cheat, for example, you should be safe.