Mobile

There are two primary ways in which you can build for mobile devices:

You can build natively on iOS, Android and other devices, utilizing the Force.com REST Web Services API, OAuth, optionally using wrappers for the native REST APIs. These can take the form of pure native apps, HTML5 apps, or hybrid container apps. The Mobile SDK supports the development of these applications.
You can use the native Salesforce Mobile client, and mobile-ready Visualforce pages.

This page provides resources for building mobile clients using these approaches.

Resources: Native Clients

Salesforce Mobile Implementation Guide (PDF).
An Introduction to Visualforce
Creating Mobile Applications using HTML5: This Dreamforce '10 session will get you started on leveraging HTML5 in your Visualforce projects.
Designing Mobile Workflow Using Apex Triggers: This Dreamforce '10 session shows how to leverage picklists, booleans, and other fields to mobile-enable advanced tasks such as calculating an invoice total and generating a quote directly from a mobile device.

Here are two code samples that utilize Salesforce Mobile:

Retrieving a User's Location from a GPS-enabled Phone - Capture the location where your mobile users enter data by retrieving the GPS coordinates from a BlackBerry smartphone or iPhone when users save a record.
Updating Salesforce.com Data in the Mobile Application - Shows how to modify related lists of Salesforce.com records in the mobile application, even when the records are not available locally on the mobile device.

Salesforce Mobile Implementation Guide (PDF) - How to set up Salesforce Mobile for iOS, BlackBerry and Windows Mobile devices.
Visualforce Developer's Guide (HTML | PDF) - Reference for Visualforce, a tag-based markup language that allows you to build custom user interfaces for your apps, including pages for Salesforce Mobile.