Part of the next-generation DisplayPort industry standard, the new Mini DisplayPort can drive up to a 30-inch widescreen display and is designed to complement HDMI. The Mini DisplayPort is just 10 percent the size of a full DVI connector.
DisplayPort is a digital display interface standard (approved May 2006, current version 1.1 approved on April 2, 2007) put forth by the Video Electronics Standards Association (VESA). It defines a new license-free, royalty-free, digital audio/video interconnect, intended to be used primarily between a computer and its display monitor, or a computer and a home-theater system.
DisplayPort has an advantage over HDMI in that it is royalty free which makes it cheaper to implement (the HDMI royalty is 4 cents per device). DisplayPort's largest supporter is Dell which has released two computer monitors that support DisplayPort and HDMI.
Update: Ann R. Thryft wrote a detailed comparison of DisplayPort and HDMI for Electronics Design News (EDN) last month.