If you are needing a soft keyboard for your new T-Mobile G1, Android has released a roadmap which include the soft keyboard feature on Q1 2009. When the Input method framework and Input method engines is complete, you will be able to use the soft keyboad from Android, plus application developers can develop new soft keyboard application for the Android OS.

Android Q1 2009 roadmap:

Input method framework (IMF)
This feature will enable support for input methods other than physical keyboards, for example soft keyboards. IMF will also enable application developers to provide IME (see below) applications based on the framework.Input method engines (IME)
IME will support soft keyboards, a dictionary of suggestions, and a suggestion algorithm. The Android platform may contain few reference IMEs, and developers can provide IME applications through the Android Market.

Source [Android] via [engadgetmobile]