<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} "!(android|blackberry|googlebot-mobile|iemobile|ipad|iphone|ipod|opera mobile|palmos|webos)" [NC]
RewriteRule ^$ http://www.example.com/ [L,R=302]
</IfModule>
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} "android|blackberry|googlebot-mobile|iemobile|ipad|iphone|ipod|opera mobile|palmos|webos" [NC]
RewriteRule ^$ http://m.example.com/ [L,R=302]
</IfModule>
REFERENCES
http://www.howtoforge.com/apache2-how-to-redirect-users-to-mobile-or-normal-web-site-based-on-device-using-mod_rewrite
