In oneiric we introduced PostgreSQL 9.0 (earlier releases had 8.4). However, 9.1 RC1 was released this week, the final is expected to be released in the next few weeks (probably with very few changes). The data format and ABI is absolutely frozen now, i. e. the same rules apply for fixes than for post-release fixes.
I uploaded 9.1 to Debian unstable now, and switched over the metapackages to point to -9.1. I am going to drop postgresql-9.0 from unstable/testing in the next few weeks, once the handful of rdepends has been migrated over. I already discussed that with the individual maintainers of these packages, and we agreed to get this done very quickly.
(it only has a few answers, as most of the replies were held in moderation due to having too many CC:s, sorry).
As 9.0 has never been in any Debian or Ubuntu stable release, I would like to drop it completely from Oneiric and move to 9.1. This will be the version that we are going to support in the next LTS, and having one major postgresql version less to maintain would be significantly easier.