Although most of the latest handheld devices (iPhone, iPad, Blackberry, HTC etc.) display full web sites in all their glory, the majority of older mobiles do not. Websites on mobiles usually render terribly due to the smaller screen and poor load speeds, as such, it's always a good idea to create a version of your site just for handheld devices. Luckily, with SilverStripe this is super simple, and here's how it's done:
Firstly, download the module from here: http://silverstripe.org/mobile/
Upload this to your root directory and run: dev/build
This will automatically set everything up for you, please note that your themes folder probably isn't writable, so you'll need to upload the "theme" folder from within the mobile module directly to your themes folder online. You may also want to rename it to something more user friendly too (E.G - MySite_Mobile)
Next, open up your CMS and click the site name on the left hand side to view the overall site config. There's a nice new tab called "mobile" click that to manage your mobile version. You can either redirect all mobile users to a sub-domain, or simply load the site with the mobile theme for them, and keep the URL the same.
We prefer the latter option, so just click "Mobile users see mobile theme, but no redirection occurs" then delete all of the content within the "Mobile domain" field. (otherwise it conflicts with the main site a bit, this bug will probably be ironed out soon)
Then choose your mobile theme from the drop down box, and save. Now you're all set!
Customise your mobile theme templates and CSS as you wish, you may also need to add templates to it from your blog theme etc, and then amend them so that things like the sidebar (that aren't really required on mobile) won't be loaded in.