I can attest to the fact that, while Ximian GNOME itself works on a Testing system, many of the software packages do not. gnomeprintbin has been broken for months, terminally and obviously so, which prevents the installation of much of the more recent packages of software that testing only has old versions, like GEdit and AbiWord. There’s a fixed version of libgnomeprint-bin available and in Unstable, but its not been moved to testing yet.
There’s also conflicts with software like Dia (where Debian packages it one way and Ximian another, completely incompatible way)
So in short, if you want to use a recent version of GNOME with Debian Testing, either using Ximian packages or not, you’re going to have to use an editor other than GEdit and a wordprocessor other than AbiWord. (GEdit 0.5.4 is rediculously unstable – it segfaults when applying changes to most of the things in the preferences window!)