Strange startup error

I have had postgres working perfectly up until recently. Now suddenly I
get an error when trying to run postgres:

% psql
psql: FATAL: No pg_hba.conf entry for host localhost, user postgres,
database postgres

% postmaster &
LOG: could not bind Unix socket: Address already in use
HINT: Is another postmaster already running on port 5432? If not,
remove socket file "/tmp/.s.PGSQL.5432" and retry.
WARNING: could not create Unix-domain socket
FATAL: could not create shared memory segment: Cannot allocate memory
DETAIL: Failed system call was shmget(key=5432002, size=3768320, 03600).
HINT: This error usually means that PostgreSQL's request for a shared
memory segment exceeded available memory or swap space. To reduce the
request size (currently 3768320 bytes), reduce PostgreSQL's
shared_buffers parameter (currently 300) and/or its max_connections
parameter (currently 50).
The PostgreSQL documentation contains more information about
shared memory configuration.

Anyone have an ideas?

-Andrew Stewart