Question: Is there a way to have an app write a into a file and then have another app read from that file?

I mean writting into the file system of the iPhone.

I want to do this without using an internet connection. So uploading the file and then downloading from the other app is not what i mean.

Thanks!

Answer: 
It appears that there are ways you can, but it won't get approved by Apple if you do.

See <http://blogs.oreilly.com/iphone/2008/09/sandbox-think-like-apple.html> for more information.
I'm not sure exactly what circumstances you're in here, but you could register app B as a handler for a particular protocol, and have app A try to open such a link (e.g. appB://yourinfohere). It depends on how much data you want to transfer and many other things, but it might work.