This has been implemented by the third party SMS Auto Respond Plug-in, which is available on the Android Market. https://market.android.com/details?id=com.suttco.locale.telephony.sms

A third party developer has released the Locale CarrierID Plug-in on the Android Market, which implements part of this feature request. This plug-in can detect which carrier the device is currently connected to.

As far as toggling data roaming as a setting, that isn’t possible due to Android security restrictions. Specifically, the data roaming setting is inaccessible to third party applications like Locale because it is stored in Settings.Secure. The developer documentation is here http://developer.android.com/reference/android/provider/Settings.Secure.html#DATA_ROAMING

The idea is that Google doesn’t want some malicious app enabling data roaming without the user’s knowledge, as users wouldn’t be happy about huge data usage bills!

The alternative is using something like APNDroid (which has a Locale plug-in), in order to disable data connectivity. This doesn’t change data roaming; it blocks all data. The end result may be similar, but the actual implementation is different.