Yes, I'm starting to think it's a private offer made to a select number of affiliates. It just seems so ubiquitous that I thought one of the major networks would carry it.
I can't remember when I first started seeing it (sometime last year), but it's been pushed for many consecutive months on a lot of big content networks, so -someone- is buying some serious media to deliver it.
There isn't an hour of the day or month of the week where it's not being delivered at ultimate-guitar.com. (That's like $150,000 for a 50MM impression insertion order at $3 CPM.)
Also, there is stricter compliance with opening up premium sweepstakes campaigns to affiliates than with standard rate sweepstakes or premium non-sweepstakes like ringtones or crush. I wonder how I know that.