Now I actually work for t-Mobile here in the UK, and can confirm that when a contract is connected to the network with a G1 it is put onto the Web 'n' Walk Plus plan as standard, which is normally an extra £7.50 per month. This plan allows you to use your mobile as a tethered modem for your computer, as long as no VOIP programs are used. The reason this is added is simply because the 1GB allowance from standard web n' walk is not enough for a device that is always connected to the internet, and whilst there are no run-on rates on T-Mobile in the UK there is throttling.
Now what confuses me is that there has been no internal comment on the ability to tether your phone to your laptop for this, even though the company is theoretically £7.50 out of pocket each month (More, as web 'n' walk is currently free on the Flext plans) I'd hardly be surprised if T-Mobile had decided to crack down, but there's nothing about it internally at present., at least not in the UK.
A more likely cause is either T-Mobile US has thrown its toys out of the pram, but I am leaning towards this being linked to the 'G2' and Vodafone. Their unlimited internet plans do not allow tethering, and I assume they are unlikely to change this. Believe me, T-Mobile are all over the idea of tethering at the moment and would almost certainly be happy to allow it for a small fee, as it essentially pure profit.
Anonymous, for very obvious reasons...