I want to use my phone as a wireless router. Since I am on a time-based data plan, I want the phone to automatically shutdown after a specified amount of time (or at least close the data connection after a specified amount of time). Basically, I want to prevent to exceed the data limit, and the shutdown is a plus. The phone is running gingerbread but I will probably install cyanogenmod 7.2.
There are many 3rd-party apps available on the Market that allow you to set up actions (toggle cell data on/off, for example) at specified triggers (e.g. on a timer.) Tasker and AutomateIt are great examples.
If you have a specific data limit on your plan (e.g. 3GB per billing cycle), a more advanced app like 3G Watchdog would be a better choice. It allows you to set and monitor limits on cell data usage, and disable it when the usage gets to the threshold.