So yeah, I finally received my invite today! (I had signed up a few hours after it was announced.)
Indeed, you can store 20,000 songs--NOT 20 GB. To be clear, my ~15k song collection is probably over 80 GB. So, yeah. There's a huge difference between saying 20k songs and 20 GB of songs.
I just hope the prices are reasonable, when it comes time to pay the piper. :/