I've just been on the FGW▸ website and I can't see details of the line closure at the end of May. They're still giving details of the 2010 closures. I thought the point ofwebsites was to give up to date info.
(To be fair I've been rightly critiscised before for missing something on the FGW website, so if I'm wrong with this gripe, apologies, but it's not obvious)
No, you haven't missed anything on the FGW website. The page hasn't been changed since last summer after details of the weekend closures for track renewal at Evesham were deleted. Network Rail's site is better, though it doesn't really make it clear that what's happening at the end of May and in mid-August are full or partial line closures, but it gives the dates. It suggests going to the FGW site to sign up for updates, but that facility has disappeared from the FGW page - and I seem to recall only ever receiving one email update anyway, back in the summer of 2009.
Network Rail are generally making a better fist of telling people what they are up to. This newsletter - which does make the closures clear - has been distributed to stations along the line in the past few days
http://www.networkrail.co.uk/documents/Projects/10995_Cotswold%20Fact%20Sheet.pdf but obviously working out what trains and replacement bus services will be running and letting passengers know about them is not their job.