Starting postgresql service and specifying data directory
There error is telling ... The problem is that you ran initdb as your local login, but I'm pretty that your login (username) doesn't have access to /var/run/postgresql. If you have a clean install, may I recommend that you remove the data directory and start all over, this time by su-ing into the postgres account:

username@jupiter:~$ sudo su - postgres


Then you can initdb

postgres@jupiter:~$ /usr/lib/postgresql/8.4/bin/initdb --encoding=UTF8 --pgdata=/mydata/pgdbdata/


Also, as Catcall mentioned, make sure you're using the correct location for your data.