That hasn't been tested in court though, has it. David Slater certainly claims that to be the case, and there are good grounds for believing that a UK court probably would uphold it - but it hasn't actually been tested in court. Until it is, it's surely still a grey issue.
Well, again it's a bit of a grey area and he would need to test it in a US court, but I think he would lose. The Berne Convention provides for rights of copyright owners in one country to be upheld in other countries, but - crucially - only if those rights are based on the copyright laws applicable in each country. In other words, if a work qualifies for copyright under the rules of country A but not under the different rules of country B, then any copyright granted in A is not required to be honoured in B. The difficulty Slater has is that it's not obvious that the image would qualify for copyright if he were a US citizen in the US, and therefore it's not obvious that any rights he has would be upheld in the US.
There's a very good discussion of the legal issues
here.