How can I get the Oracle TNSListener to start after changing my host name? When running 

lsnrctl status


I can see that it is using the old host name, but I don't know where it is pulling that value from.

My Oracle installation is 11.2.0 on Windows7 x64, if that makes any difference.
When you run lsnrctl status, one of the things that it tells you is the location of the Listener Parameter File (the listener.ora file)

LSNRCTL for 64-bit Windows: Version 11.2.0.1.0 - Production on 06-JUN-2012 13:12:22

Copyright (c) 1991, 2010, Oracle.  All rights reserved.

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=EXTPROC1521)))
STATUS of the LISTENER
------------------------
Alias                     LISTENER
Version                   TNSLSNR for 64-bit Windows: Version 11.2.0.1.0 - Production
Start Date                06-JUN-2012 09:33:58
Uptime                    0 days 3 hr. 38 min. 24 sec
Trace Level               off
Security                  ON: Local OS Authentication
SNMP                      OFF
Listener Parameter File   C:\oracle\product\11.2.0\dbhome_1\network\admin\listener.ora
Listener Log File         c:\oracle\diag\tnslsnr\jcave8560w\listener\alert\log.xml
Listening Endpoints Summary...
  (DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(PIPENAME=\\.\pipe\EXTPROC1521ipc)))
  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=127.0.0.1)(PORT=1521)))
Services Summary...
Service "CLRExtProc" has 1 instance(s).
  Instance "CLRExtProc", status UNKNOWN, has 1 handler(s) for this service...
Service "jcave11gR2XDB" has 1 instance(s).
  Instance "jcave11gr2", status READY, has 1 handler(s) for this service...
Service "jcave11gr2" has 1 instance(s).
  Instance "jcave11gr2", status READY, has 1 handler(s) for this service...
The command completed successfully


If you open that file, there will probably be an entry like this

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC1521))
      (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    )
  )


where the host is specified.  Change the host there and restart the listener.