Live CERN Higgs Announcement in 20 Mins 
So, the magic number here was 5σ, the generally accepted "gold standard" for discovery, which would mean a 1 in 1.74 million chance that the results occurred by chance rather than being a signal. As other commenters have pointed out, the presenter originally announced a 4.1σ observation, then continued to add data from other experiments until the combined result was 5.1σ. However, right at the end, he added in some additional data which actually brought the significance down to 4.9σ... That's science - you can't ignore data just because it ruins your big presentation.<p>IANAPhysicist, but I'd be interested to know how strict the 5-sigma discovery rule is considered - for example, could they still get a Nobel prize for a 4.9σ announcement? I suppose it's not that big a deal - the LHC is still running, and I'm sure they'll have enough data for a true 5σ announcement soon. Regardless, hats off to all involved, it must be exciting to be at the forefront of human knowledge :)