version:
tried python 2.6 on rhel55
tried python-2.7-8 on fedora14

Steps to Reproduce:
1. install machine, have a fixed hostname up to mac address
1.1.1.1 = machine.something.com

2. wait a week or longer when your local dhcp server changes your IP but not
hostname
1.1.1.159 = machine.something.com

[root@machine.something.com ~]# host machine.something.com
machine.something.com has address 1.1.1.159

ifconfig shows the same

3.   gethostbyname(gethostname()) shows wrong IP, the original one

Actual results:
[root@machine.something.com ~]# python
Python 2.4.3 (#1, Jun 11 2009, 14:09:37)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-44)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from socket import gethostname
>>> from socket import gethostbyname
>>> gethostname()
'machine.something.com'
>>> gethostbyname(gethostname())
'1.1.1.1'
>>>

Expected results:
gethostbyname(gethostname()) will return your actual IP, 1.1.1.159

Other:
1.1.1.1 = machine.something.com is added in /etc/hosts in the time of OS installation , then its not changed

This is not a Python issue.  gethostbyname() and gethostname() are just a wrapper for the OS's functions of the same name; you'd get the same result when coding the example in C.