Debian
Stable (current release) and unstable are always supported: we do release builds and occasional updates.
Oldstable is supported for a while after a new major stable release. Support will be dropped no later than the end of security support (for squeeze, that date is 2014-06-15, that being one year after wheezy was released), and likely several months sooner.
Proper support for testing may be missing while oldstable is still supported, but it is otherwise supported.
i386 and amd64 are currently supported. We expect to add x32 at some point.
Ubuntu
We will, primarily, be building for the current release, the previous release and the current LTS release. There will be a few months' overlap with the previous LTS release; otherwise, obsolete releases will be dropped fairly promptly (with our first release following the new Ubuntu release).
We may start supporting the next release a month or so before it's due, and we expect to do so regardless in the month in which it's due. However, this largely depends on deboostrap in Debian testing/unstable recognising the new release.
i386 and amd64 are currently supported. x32 may be added; status is unknown.
