It could have been a last minute licensing issue in the U.S. I remember that's what kind of held up the SingStar release in Europe. Thankfully the U.S. had to wait a few months for it, since it allowed the SingStar store to build (when it came out in Europe, there were only a few songs or so to download).
I just picked this up last night for $53 at Costco (I missed out on the GS deal) and so far I love it! The Singstar online stuff is pretty slick, and I bought 11 songs last night and didn't have a single issue with the store. It took about 20 mins to download them all.
The one thing I don't understand is the availabilty of certain songs, or lack of. I would love to get Fix You by Coldplay, or Feel Good Inc. by the Gorillaz, as they were removed from the USA release. They should have these songs on the store!
Not sure if people have seen (this is mostly Europe, but has many of the same songs) this:
You can preview various songs online on that site.