Apple has alerted AOL subscribers who use their credentials to access iTunes and other Apple services that they will need to switch over to an Apple ID for access as of March 31.
And yes, according to AOL's latest financials, there still are more than 2 million domestic subscribers of its services. Subscribers have been able to use their same AOL credentials to access iTunes, the Apple App Store and iBooks store.
Apple states on its support page that "You must convert your AOL Username to an Apple ID in order to maintain access to the stores and to content you purchased previously."
Based on the wording of Apple's warning, it sounds as though AOL instigated this change ("AOL will no longer allow..."). Apple provides steps that AOL subscribers can use to transition to the new iTunes, etc., logins.
Apple and AOL first partnered for iTunes access in 2003.
The 9-to-5 Mac blog includes a copy of the letter Apple sent to AOL subscribers who access Apple services via their AOL credentials.