Instat is pushing a report on Mobile Music in Asia/Pacific (for $2995!) with excerpts like this
Ringtones have been the primary driver for mobile music growth in the past, but this will change as new mobile phones equipped with digital music file playback capability create a new market.
The size of South Korea’s mobile music industry has already surpassed the country’s conventional music industry.
What they don't discuss in the press release is the model for mobile music sales in Korea. Typical is SK Telecom's MelOn service where users have the option of paying a flat 5000 won (US$5.24) per month for unliimited access to any of over 700,000 songs. Pyramid has a good description of the service although their subscriber figures are dated. This month's Red Herring (in an article on Big Music vs Steve Jobs) has these figures for MelOn subscribers,
About 600,000 of MelOn’s 4 million-plus users are $5-a-month subscribers getting unlimited music via PCs and phones.
Pyramid claims "the key catalysts to successful mobile music models will be 3G+ wireless technologies, flat rate bandwidth pricing and extensive libraries."
I would also note that, as with other services in Korea, they provide ubiquitous access, i.e. the same service works on phone or PC.
But I claim the key difference (versus US & EU mobile music services) is a reasonable total cost that can be easily understood by customers.