If the video / movie files you want to view are in a format that is not supported by iPhone, you may be able to convert them into a compatible format with Video Converter Pro for Mac.

The iPhone is an Internet-connected, multimedia GSM smartphone designed and marketed by Apple Inc. Because its minimal hardware interface lacks a physical keyboard, the multi-touch screen renders a virtual keyboard when necessary. The iPhone functions as a camera phone, a portable media player, and an Internet client. The first-generation phone hardware was quad-band GSM with EDGE; the second generation added UMTS with 3.6 Mbps HSDPA; the third generation adds support for 7.2 Mbps HSDPA downloading but remains limited to 384 Kbps uploading as Apple had not implemented the HSPA protocol. 
Apple announced the iPhone on January 9, 2007, after months of rumors and speculation. The original iPhone was introduced in the United States on June 29, 2007 before being marketed worldwide. Time magazine named it the Invention of the Year in 2007. Released July 11, 2008, the iPhone 3G supports faster 3G data speeds and assisted GPS. On March 17, 2009, Apple announced version 3.0 of the iPhone OS operating system for the iPhone (and iPod Touch), released on June 17, 2009. The iPhone 3GS was announced on June 8, 2009. It was released in the U.S., Canada and 6 European countries on June 19, in Australia and Japan on June 26, and will see international release in July and August.
DVD to iPhone Converter for Mac is an easy-to-use Mac DVD ripper software to get movies onto your iPhone. You may use DVD to iPhone Converter for Mac to rip encrypted movie DVDs and enjoy movies on your iPhone. The converted MP4 video can be used for playback on iPhone 3GS, iPhone 3G. Also, the video files can be used for video editing.

(PC user? Get Windows version here)