While I enjoy Elvis songs (especially his early stuff), I understand the absence from New Vegas (based on the cost of using any of his songs). It's going to be a long time before Elvis songs are in the public domain. Take a look at the Wikipedia article on http://en.wikipedia.org/wiki/US_copyright_law. It isn't 20 years:
Before 1972, sound recordings were not subject to federal copyright, but copying was nonetheless regulated under various state torts and statutes, some of which had no duration limit. The Sound Recording Amendment of 1971 extended federal copyright to recordings fixed on or after February 15, 1972 (the effective date of the act), and declared that recordings fixed before that date would remain subject to state or common law copyright. The Copyright Act of 1976 maintained this until February 15, 2047, which was subsequently extended by the Sonny Bono Copyright Term Extension Act to the same date in 2067.[29] As a result, no sound recording can reliably be considered in the public domain in the United States before that date, even if the recording was in existence before 1923 and even if it originated in another country where it has entered the public domain.[30]
So unfortunately for Elvis fans, his recordings won't enter public domain for a good while. Maybe by the time Fallout 50 releases.....10 years before the War