App to block data usage on iphone I was wondering if it was possible to block data usage completely when a certain data transfer limit has been reached? Does an app exist for that? 

I am aware there are some apps that allow users to view the data usage, but is there an app to prevent completely from using Data when limit is exceeded, automatically?! This would be especially usefull to prevent the extra fees when exceeding my 1GB data usage limit.
No, that is not possible: it would mean Apple allows developers access to deactivating data access. I guess you can understand the security problem that would mean: an app could decide on its own you don't have data access anymore.

The only way to do that is to regularly check your data usage in “Settings > General > Usage > Cell network usage” (menu names may not be the exact ones, my iPhone isn't in English, sorry). You can reinitialize these statistics each time your limit is reset.

If your limit is crossed, switch data off in “Settings > Network” (iOS 5 only).