There are a few ways you can build a website based on mobile videos. You can pick a niche such as funny videos, video game content, trailers etc. You can convert these videos yourself using Xilisoft or another converter and hosting the files yourself. But this way will cost you loads for storage fees and bandwidth.
Therefore I suggest you uploading videos to VidSkip.com or Videobam.com. Both of these sites will encode your videos to mobile format that works on hundreds of mobile devices such as iPhones, Blackberry, Android phones, Windows phones, etc.
Right now I am making thousands of dollars each month by making niche mobile video sites. I use ad networks like Jumptap, Admob, Tapit, Nexage, Smaato, & vDopia to monetize my traffic. I believe that it is truly an untapped market and many people can make hundreds if not thousands too.
Here are some stats of one of my websites to show you guys the potential of this niche: