Although there are a lot of answers already, I'll chime in with xml2.
$ xml2 < test.xml
/config/global/install/date=Tue, 11 Dec 2012 12:31:25 +0000
/config/global/crypt/key=70e75d7969b900b696785f2f81ecb430
/config/global/disable_local_modules=false
/config/global/resources/db/table_prefix
/config/global/resources/default_setup/connection/host=localhost
/config/global/resources/default_setup/connection/username=root
/config/global/resources/default_setup/connection/password=pass123
/config/global/resources/default_setup/connection/dbname=testdb
/config/global/resources/default_setup/connection/initStatements=SET NAMES utf8
/config/global/resources/default_setup/connection/model=mysql4
/config/global/resources/default_setup/connection/type=pdo_mysql
/config/global/resources/default_setup/connection/pdoType
/config/global/resources/default_setup/connection/active=1
/config/global/session_save=files
/config/admin/routers/adminhtml/args/frontName=admin
With a little magic you can even set those as variables directly:
$ eval $(xml2 < test.xml | tr '/, ' '___' | grep =)
$ echo $_config_global_resources_default_setup_connection_host
localhost