The web page at http://www.postgresql.org/developer/alpha says:
The current alpha is 9.1alpha2, the second alpha for version 9.1.
This is surely not right, since 9.1alpha3 happened some time ago.
That web page also has links to 9.1alpha1 installers, which seems a
bit obsolete at this point.
I just packaged alpha4, so this web page is shortly going to be even
more wrong, although I'm not entirely clear what I need to do to get
it to show up here:
http://www.postgresql.org/ftp/source/
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company