I don't believe this to be true just yet.
We have not yet had a SINGLE word of confirmation from the band's management, label, or members.
Lots of people have been meeting the members of U2 at Abbey Road studios and book signings in the past few weeks, and have been asking them about a new album being released.
If this Greatest Hits was true, I'm sure they would have let something slip, especially if it's out next month.
Just for the record, though, I think it would be a very unnecessary release, with the period 1980-2000 already being covered.