For now, the option to purchase unprotected files will only apply to artists under the EMI label. Wikipedia lists many of the artists signed to the record giant. While it'll be great to have fully unrestricted access to music in the future, this is a great first step for PSP owners that want to purchase their music legally from a service other than Sony's failing Connect store.
See also:
Comprehensive guides for making your PSP a better music player
Stream music from your PC to your PSP
