For whatever reason, I needed to upgrade my apt-get-installed copy of Postgresql from 8.1 to 8.3. Now, I can understand why, from a sysadmin viewpoint, or from the maintainers view, 8.3 is not as stable as previous versions. However, as a user, or as an application developer, 8.3 is a much better option. It is also certainly reasonable to want to use the new stuff available in a new version. Other system choices enable me to use the software I want or need. I've been running 8.3 for months on Mac OS X.
On Debian Etch it is generally a good idea to add the backports repository to the list of sources for apt to use. This gave me the version of Postgresql that I wished to use but, unfortunately, it fails to uninstall all of the common libraries for the client and server, as well as the dev library (libpq-dev) for the appropriate version. This failure in upgrading is due to the ability of Postgres to run multiple versioned "clusters" at the same time. I however, uninstalled (without doing --purge, my mistake) version 8.1 and installed 8.3 and was not trying to run them together.
The reason this caused problems for me was the native postgres gem for ruby was compiling itself with the 8.1 headers and so, data_objects and dm-core would refuse to connect to 8.3. Especially infuriating was that I could see the server running and could connect with the psql client using the same connection information I was providing to DataMapper.
The solution was to uninstall everything postgresql related using apt and the --purge flag. Taking special care to include libpq4 and libpq-dev in that list. Uninstall the postgres gem, as well (and possibly data_objects and dm-core).
Now add "deb http://www.backports.org/debian etch-backports main contrib non-free" into /etc/apt/sources.list and do sudo apt-get update to pull the list of packages. Now reinstall postgresql-8.3, libpq5, and libpq-dev and be sure to pass the -t etch-backports option so that all the required libs get pulled from that repository.
Finish up by reinstalling all the required gems and all should be well. Note that my experience here is not typical, and in general, being strict about package stability is generally a good cause. However, it is a very useful to have backports available when needed and the packages there are generally more stable than other random repositories.