The current release of TurboGears, 0.8.9, TurboGears should be considered alpha software. This means that there can be breaking API changes between now and 1.0. Major strides are being made to help TurboGears reach 1.0 quickly. Stable versions of TurboGears' included projects are now in use, and we are working on some great features for 0.9. The 0.9 alpha release will be a stepping stone to 1.0. Once people have had a chance to work with 0.9, we'll make necessary tweaks and release 1.0.
It is worth noting that after 1.0, breaking API changes will only come in major point releases and will be documented. Wherever possible deprecation warnings and automated migrations will ease the transition for backwards incompatible API changes. TurboGears 1.0 will be a stable platform to build on.