This is implemented by the Locale Google Voice Plug-in, available on the Android Market.

Apps cannot directly change call-forwarding settings on the phone, due to security restrictions in Android. Despite this, there are two workarounds to allow for a call forwarding setting:

1. The Locale Dial Plug-in setting can be set up to dial any number. Most cell phone carriers allow special dialing codes to enable/disable call forwarding. For example in the United States dialing *72 followed by a phone number will enable call forwarding to that number. Then dialing *73 later will disable call forwarding. Please consult with your carrier for its specific dialing codes. https://market.android.com/details?id=com.mariobialos.LocaleDialPlugIn

2. Use Google Voice. The Locale Google Voice plug-in can be used to change Google voice call forwarding settings, although your callers must be dialing the Google Voice number for this to work. https://market.android.com/details?id=com.steelgirder.LocaleGoogleVoicePlugin