The User Interfaces that have been added to APEX 4.2 have a few properties to help define what URL should be resolved - hence whether you see a page designed for a mobile or desktop.

After testing these to determine behaviours as the values change, I find it would make an easier flow chart to follow.

User Interface Properties

If Auto Detect is off, it will render the default UI
If Auto Detect is on, and hardware matches UI, this will override default.

Should auto detect fail, or no default are specified when auto detect is off for both interfaces, APEX will display the following page.

Page seen when no default and auto detect is off

This page was also displayed when auto detect was on for both UIs on a Samsung tablet, so probably best to define at least on default.

Order of the list is defined by UI Sequence - which is the only display this property seems to affect - nothing within the builder.

This screenshot captures the default appearance, but you can override the stylesheet used by using the User Interface Detection CSS File URLs property, under application properties.

ADMIN 01 - User Interface Selection

Please select a user interface from the list below:

jQuery Mobile Smartphone

Desktop

Media queries are also accetable

[media="only screen and (max-device-width: 480px)"]/mycss/smartphone.css

So those with good CSS kung fu could really make that a page professional looking entry point, should your application require it. Of course, you could build your own page using the dictionary view  APEX_APPL_USER_INTERFACES, and make it look however you want declaratively.

And of course, the page that initially opens will be the relevant home page URL for the user interface used, and the relevant login page should authentication be required.

The exact mechanics behind the detection is not known - David Peake wasn't sure off the top if his head when I asked him about it in Melbourne. If anyone knows, I'd be interested.
What he did suggest, however, is that Tablets will be available as a User Interface option in a future APEX release. I predict perhaps SmartTVs will also be a future option.

Scott