openoffice.org doesn't require openjdk-6-jre perse its more that there have been known problems getting it to work with sun-java6-jre and it is also non-free so isn't recommended to be used. openjdk-6-jre is an open source project run by Sun themselves so is likely to be at least as good as sun-java6-jre and liable to be less buggy since it is open source which means bugs can be fixed easier. There has been discussion in the past to remove sun-java6 from Ubuntu entirely since it is essentially redundant now, but it is still in Karmic for now.
openoffice.org doesn't require openjdk-6-jre perse its more that there have been known problems getting it to work with sun-java6-jre and it is also non-free so isn't recommended to be used. openjdk-6-jre is an open source project run by Sun themselves so is likely to be at least as good as sun-java6-jre and liable to be less buggy since it is open source which means bugs can be fixed easier. There has been discussion in the past to remove sun-java6 from Ubuntu entirely since it is essentially redundant now, but it is still in Karmic for now.