mack - 5:14 am on Dec 28, 2010 (gmt 0)
My opinion or .mobi is not great. On a handheld device the keyboard makes typing a lot more difficult. Make the domain as short as possible. .mobi in its self is longer than it needs to be.
if I had to choose between
I would choose the latter. Because this is a sub domain it retains your existing name/branding etc but will be viewed as a site in its own right.
I think the key here is how you present the mobile site to your users. With modern smart phones they can handle full web pages with no problems. Older phones may struggle.
One method that is widely used is to sniff out mobile browsers and redirect them to your mobile site. If you do this I recommend you provide a link to allow the user to choose to return to the main site.