Display a Block for mobile only without creating new theme I'm trying to display a main navigation menu block but ONLY for the site when it's viewed on a mobile. I need to do this without creating a new theme to do so though so it all works on 1 theme only.

Any ideas how this can be done?

Thanks
Take a look at the Browscap Block module. Here is a quote about it (from the module's project page):

... adds visibility options to block configuration settings to allow you hide or show blocks in mobile devices.
If you are looking for the same thing for Panel panes check out Browscap CTools.