i actually think the b-side disc is even less likely than a double album, given their propensity to hold back material for a later date if it doesn't fit the album.
and why would a band wait 5 years to realease an album with an optional bonus disc of the rest of the stuff? that's like saying, "oh yeah and here's the other junk we recorded too" and bring them back down to square one again.
most likely: single album, perhaps a few tracks longer than their typical stuff (maybe 15 tracks! gasp!) followed by another record soon. Like Kid A / Amnesiac. or just saving those other tracks for another damned 5 years and seeing what works with the next batch of new stuff.
next most likely: double album (wishful thinking on my part)
least likely: album and b-sides disc.
I really don't see U2 going the In Rainbows Disc 2 route at all.
But then again i might be wrong. i usually am. I am just excited about the facts:
1. we are getting a record
2. it supposedly is a change in sound
that's all i really need to know to be excited.