There aren't many details, but MusicRow.com tried out a few of Sony BMG's MusicPass MP3 cards and had this to say about the experience:

This writer tested several cards including Ultimate Santana, a collection of 18 tracks and three bonus music videos. MusicPass.com began to automatically download four songs at once, but due to the large number of tracks and high file size (the videos were around 80 megabytes each) I ran out of time before all the material had transferred. Upon logging in again later, the site remembered me (via the pin/serial numbers) and picked up where it had stopped. Before long I was sweating on a spin bike grooving to “Oye Como Va” on my iPhone iPod.

Other items of note (from the post and the MusicPass FAQ:

• Each card has a scratch-off PIN code and serial number combination that are entered at the MusicPass site.
• After the consumer has downloaded the album, there is an opportunity to purchase a complete catalog album from the same artist.
• Said Sony BMG Nashville's VP of Digital Business,"They have to be activated at the register just like any other gift card purchase."
• The cards have no expiration date.
• The files are watermarked with "a unique code indicating that they belong to Sony BMG" but no personal information.
• Users are not required to enter anything other than the PIN code and serial number. An email address is optional (to receive artist info).

To watch a video about the MusicPass cards, go to the website and click on the "What Is MusicPass?" link.