The MHL specification, which stands for Mobile High Definition Link, is a 1080p HD video and digital audio interface for connecting smartphones and other portable devices (tablets, cameras, etc.) to HDTVs. So what's the big deal? It simultaneously provides power to your phone--something that HDMI cables don't do. It remains to be seen how power-hungry a dual-core, 4G phone with a 3D display will be, but given how quickly smartphones with lesser specs seem to lose power, charging is always appreciated.
The specification also allows any TV remote to control your phone's interface, apps and content. Basically, you can hook your phone up to any HDTV, by any manufacturer, and have complete access to it on the big screen.
This also means MHL phones will only have one port and require one cable needed for charging and hooking up to a TV. Other HDMI-enabled phones, like the HTC EVO 4G for example, have two ports: the microUSB port (for charging and hooking up to your PC) and the HDMI port. A single port allows smartphones to be slimmer and sleeker in build.
You can also use your standard USB connector with the port. When connected, your phone will automatically recognize whether it is hooked up to an MHL port or a USB port and will switch accordingly.
Wong was unsure whether Verizon would package an MHL cable with the EVO 3D or sell it separately in stores. He expects the cables to be cheaper than HDMI cables (which are generally sold separately) because they are much thinner and cost less to make.