[Mediawiki-l] Redirection to non-www domain name

[stas at khirman.com]

The configuration:
MediaWiki      1.15.1
PHP     5.2.12 (cgi)
MySQL     5.0.81-log
Website  www.wikivalley.com
Clean installation with "Short URL" rewrite rules in .htaccess
 
The problem:
 
Apparently , every URL for non-defined article like
http://www.wikivalley/com/wiki/SomePage redirected to
http://wikivalley/com/wiki/SomePage ( not omission of WWW prefix). Also
http://www.wikivalley.com is redirected to http://wikivalley.com/wiki/Main_Page
.
 
I put a workaround using rewrite rules , but every such request results in two
back and force "301 Redirect" calls ( + about 1 second latency).
 
Did i missed something in configuration?
 
Any help is deeply appreciated!
Stas