Hi, I still use my pearl to tether to the N800 every day. I got a text message the other day about T-Mobile's newest upgrade. I think its the latest version of the OS so it may OR may not work. As for the Blackberry Curve, it may be too new. I can check but if I remember correctly it was OS version 4.1.0.66 for the Pearl that worked. There is a way to downgrade or upgrade any providers (T-Mobile, AT&T and others) with any OS but I think it may be device specific. What OS version do you have for the Curve? I think I'm going to try upgrading to the newest OS again for the Pearl and try if it works. Downgrading's a chore since backing up takes some time. I'll do it though if breaks the tethering ability.
I don't have a Curve to test anything on but I know the Pearl with the older version definitely works. Good luck!

DigitalAce7