Using a different host name for mobile is not good for SEO, if it is the same site you really don't want to duplicate content at a different host name.
Forcing a preferred hostname means only one host name is really used all the other host names are redirected to the preferred host name and this improves SEO by not having multiple urls for the same content.
If you really want a separate host name for mobile that implies a second site and you can make the second site use a skin designed for mobile.
But I think it is best to not use a separate host name and not try to use host name as a mechanism for changing skins. You can use a separate skin for mobile but that is something detected from a fragment of the user agent string that identifies it as a mobile device. You can tweak the string fragments that are used in mojoPortal to detect a mobile device as mentioned in the documentation for our Mobile Kit Pro Add on.
In site settings you specify the mobile skin and if the fragment is contained in the user agent string for the device then it will apply the mobile skin.
The default setting is like this but you can copy it to user.config and modify it:
<add key="MobilePhoneUserAgents" value="iphone,ipod,android,iemobile" />
you can add or remove fragments in this comma separated string to make it detect or not detect specific devices as being "mobile" as long as you can find a unique fragment in the actual user agent string of the device. One problem with android for example is that I don't know a way to determine android phone from android tablet whereas ipad is different from iphone. For a larger tablet screen I think the main skin should be used, only for the very small screens of mobile phones do we really need radically different design.
Of course you could also use a different strategy, rather than detecting mobile devices and changing skins just using responsive design in css with media queries to adjust for screen sizes so that one skins works/adapts for all devices.