Activating the mobile theme requires configuring an Exception in the Design of the Magento Configuration.
Go to: System > Configuration > Design
Make sure that the Package is set to the package with your theme. If you’re using a different package than the default (or enterprise if you’re on that platform), then you will need to add an exception for the package. Click on “Add Exception” below the Current Package Name.
This will display a form with a few fields. The “Matched Expression” field is a string you want Magento to match in the User Agent of the browser. “Value” is what you want the field value (in this case package name) to be when that expression returns true. So, if your theme isn’t in the default package, you would put default in here (since the mobile theme is at default/iphone).
The matched expression is case sensitive. You will need a different Matched Expression for each platform. So, for example:
You would then do the same thing in the Themes area, except the value field will now be “iphone” (the name of the theme folder).
Set those, clear your cache, and check out the site on your phone. You should now see the mobile theme, and your computer should still show the main theme.