I use dropbox.com to store my script online and here's why it's a good idea:

• Files are stored on your computer AND online in your dropbox folder
• This enables you to work from several computers (I work on my desktop and laptop); and the file updates on all your computers with the dropbox software.
• It's free and free is good!

There are also several alternatives like google docs or Adobe Story (currently in beta), both very good as well.
Happy writing!