Domain switches to IP

[TheSeoGuy]

My domain changes from the domain to the IP once a visitor arrives at the website.

For example.
Typing the following into the browser: http://example.com

Results in: http://example.com

However, typing the following into the browser: http://www.example.com
Results in: http://#*$!.xx.xx.xx
(where #*$! represent the numbers of the IP)

Can anyone tell me why this is happening?

Thanks in advance.

[edited by: bill at 8:24 am (utc) on Jun 18, 2010]
[edit reason] Use example.com for examples [/edit]

[lammert]

If the domain name in the address bar switches to the IP address, there is probably some redirect code which tells the browser to do so. I would expect a wrongly configured 301 redirect to be causing this. Does this site use 301 redirects?