It has been long time since I thought of Celtx, I wrote about the free desktop scriptwriting software back in 2009. I took another look and have an update. The desktop version now comes in a free and paid, advanced feature version.
However there is now a mobile scriptwriting application for the iPhone/iPad crowd.
The mobile app isn't free. I just checked an it is $9.99 and you need iOS4 or higher to use it.
It does give you the ability to write your masterpiece anywhere, anytime. You can also sync it with your desktop application so that ought to be worth something right there.
I do not have either device so I can't speak about the quality.
There are plenty of Celtx videos on YouTube so you can get a sense of how it works. If you need a scripted or structured type of video, are a Mac user or have one of the Apple devices you are good to go.