Subdomains and main site redirects

I haven't yet installed the domainsonly plug in, can you advise what it is specifically designed to do and what if anything it will over-ride?

I figured out how to set up sub domains but now when i go to mobile.gcpmobile it looks just like my desktop.  also when i go to my desktop version on my whone www.gcpmobile.com it does not redirect to the mobile version.  also when i go to iphone.gcpmobile.com there is nothing...could you shed some light on what i have done wrong?   

mschnetzler, what are values of "Domain (multisite) support", "Domain name" and "Redirect to domain" settings (should be "Yes" and subdomain names)?