While that's a limited-time offer (presumably this means that people signing up during the offer period get unlimited music storage indefinitely, and not that they will be expected to pay for the space next time they renew), Amazon is also providing free Cloud Drive storage of all Amazon MP3 purchases, whether they were made before or after the launch of the service.
The 'unlimited music' offer is also being extended to customers qualifying for 20GB of Cloud Drive storage under previous promotions.
In related news, the Amazon Cloud Player (which streams music to a web client) has been updated to work on the iPad as well as any Mac, PC, or Android device.
"Customers are already enjoying Cloud Drive and Cloud Player and now for just $20 a year, customers can get unlimited space for music," said Craig Pape, director of Amazon Music. "Additionally, we are adding free storage for all MP3s purchased from Amazon MP3, and support for the iPad. Our customers love Cloud Drive and Cloud Player and we're excited to innovate these services on their behalf."