I wonder what actually happened... As of now Lars is hospitalized in Zurich with a "mystery ailment". I hope he's OK, but the vagueness of the article at Metallica.com isn't exactly promising.
Since the article mentioned that he was the in the hospital with one sentence and then went on for paragraphs describing the show, I wouldn't imagine it was that big of a deal.
METALLICA's Lars Ulrich has been hospitalised in Switzerland for reasons that are at this time unknown to the general public, forcing the band to show up to their Download Festival performance in the UK sans their skinsman.