Update 9/2/2104: It looks like this test never made it to full release, as iCloud users are still not prompted to authenticate with 2FA when logging into iCloud.com.
With all the security risks populating the world, two-step verification is the strong standard to have for information safety. Apple brought the process to your Apple ID last year, now iCloud and its associated apps are getting it as well. With a two-step verification users must enter a code that is sent to their device after you enter your password.
This current wave of two-step verification hasn't hit all users just yet -- as noticed by the folks at AppleInsider this morning, some users aren't seeing the prompt when they login to their iCloud accounts. You'll be able to tell if the two-step verification has been activated on your iCloud account the moment you login, as your web apps will all be greyed-out with a lock over them. Once the verification process is completed all of the apps will be accessible.
There's one exception to the greyed-out apps; Find My Phone. Given that it's possible you may have lost the trusted device Apple would be sending your verification code to, that was a wise move on Apple's part. If you login to iCloud today and nothing has changed, don't worry. You can expect to see the added layer of security hitting your setup sometime in the near future.