Well after doing a bit of reading about the T-Mobile G1 it appears that many of the pre-release reviews in the UK were not using the same version of the phone given to the customers, as they talk about features that are unavailable in the UK release. Here's what I found:
The voice dialling application you can find many comments about does not exist on the UK version of the T-Mobile G1. Any reference to it does not exist in the manual and it does not appear in any of the icons. It also does not work when you hold down the send key on the phone or the equivalent action on a bluetooth headset. T-Mobile technical support tried to help me get it working, but all they did was refer to the manual to read the instructions and could not find any - they could not xplain why it does not exist.
Many of the comments about the voice dialing do not present a pretty picture, so maybe Google disabled it as it did not meet their standards.
The IM application shows similar restrictions: you cannot use the non-Google instant messaging clients, only Google Talk. No the only reason I can think of the unavailability falls with T-Mobile: they have never been keen on IM (even worse on SIP/VoIP), so maybe they wanted the restriction? The Accounts List screen offers no functionality: the menu key does nothing and a long press alos gives no response.
As you can see I can think of a few ideas why, but I would love for an official answer. I would love to know where the T-Mobile G1 users should go for support - T-mobile cannot give the right answers, so where?