What causes the "Unable to download application" error on an iPod Touch? On some wireless networks, whenever I try to download or update an app I get the following message: 

Unable to download application.
&lt;Your App&gt; will be available for download when you log in to the
iTunes store on your computer.


I assume it is some kind of firewall issue. Assuming it is, does anyone know what firewall ports are required to download apps via WIFI to an iPod Touch?
I had this twice on my iPod touch.

When it happened first, there was not enough free memory on the iPod.

The second time it was an app (yxplayer) not designed for the iPod. I even could not install it after I had downloaded it in iTunes. If I remember correctly iTunes told me that this app can not be run on an iPod touch. After i "upgraded" my iPod to an iPhone 4, I could install the app without any problems.