I think the very fact that the following is mentioned underneath the order form says it all...
These tickets may be subject to a change of date. If this unlikely scenario does occur all tickets will be valid for the subsequent dates. The Online Ticket Shop will inform you of any date change
Its clear from this that, however likely the shows are to take place (and I personally think they will), they are obviously
STILL UNCONFIRMED. U2.com will officially announce dates I'm sure, and I'd advise everyone and anyone and his dog to wait until that point. Does anyone remember a similar thing in 2002 when U2 were supposedly going to do a European stadium tour with Oasis opening for them starting in Amsterdam? U2 hadn't announced anything, even if it was planned, and lots of people were scammed into buying tickets. I'm not suggesting this one is a scam also, but it doesn't exactly seem conventional does it?
Even after waiting, I'd still avoid those kind of prices. I think U2 would be pretty disgusted with £275 a ticket. There are always ways to get hold of tix at reasonable prices if you miss out when they go on sale, be it through EBAY, people on here, or the various U2 groups/lists etc. Communication!
Even turn up at the venue itself without a ticket - not that I'd personally recommended that - but if you're willing to chance your luck - I've never been to a show yet without someone selling tickets outside at a reasonable price - and that includes 17 U2 shows now.