iPhone 4 - How to automatically launch a script every fixed time. (Plist)

The goal is: launch my own script on my iphone every 300 seconds (5 minutes) for example.

Since the iPhone has no working cron deamon since 3.0 you can achieve it with the launchd deamon.
Launchd on Apple manpages
You have to have a running script, I suggest you to test it on iphone shell before going forward.
For example the script to launch could be this:

/var/root/scripts/hello


You have to create a .plist file in /System/Library/LaunchDaemons/ , I called it org.me.hello.plist
The hello.plist should be like this:

 <?xml version="1.0" encoding="UTF-8"?>  
 <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">  
 <plist version="1.0">  
 <dict>  
 <key>Label</key>  
 <string>org.me.hello</string>  
 <key>ProgramArguments</key>  
 <array>  
 <string>/var/root/scripts/hello</string>  
 </array>  
 <key>RunAtLoad</key>  
 <true/>  
 <key>StartInterval</key>  
 <integer>300</integer>  
 </dict>  
 </plist>  

After that, you have to tell the system to load this new service hello.plist


iPhone4:~ root# launchctl load /System/Library/LaunchDaemons/org.me.hello.plist

Now your script should be launched automatically every 300 seconds (5 minutes) by the iPhone.

That's all.