U2-Glastonbury was announced last time round in late November. That's crazy early, and I assume was only because the Glastonbury people were a bit beside themselves. They never do that.
Even getting confirmation of the headliners in February is pretty early, it's usually not till April-ish that they formally confirm that kind of thing. I think in this case, the artists got ahead of the festival. It was 'announced' by the Holy Moly gossip site that it was definitely U2/Coldplay/Beyonce, and then Coldplay and Beyonce's people confirmed. U2 haven't, even though everyone 'knows' its U2 - its the worst kept secret ever.
So why didn't they follow and confirm? Easy thing to do, and its nothing surprising, they wouldn't be pissing anyone off. I think its probably more evidence that they are, in fact, up to something, and they're just holding fire on any/all publicity?
On its own, U2-Glastonbury isn't news - it was essentially confirmed when U2.com advertised the ticketing info. But they can properly confirm it now, or... its another nice bullet point alongside other news to come?