I have a T-Mobile UK plan with Web N Walk Plus, which allows tethering to laptops instead of carrying around a USB Broadband Stick. It amuses me that O2 are charging for a Tethering Bolt-On, more than what you could purchase a Three Mobile broadband plan for, and what might actually be more convenient ... anyway I digress.

Ok, to run a non-O2 sim card in a UK purchased iPhone, you will need to Jailbreak it and then SIM Unlock it.  Jailbreaking, essentially means that you are bypassing Apple's control over what software is installed on your handset.  Jailbreaking needs to be done to allow the SIM unlock software to work.

DISCLAIMER: Information provided here is on an "as is" basis, and I will not take responsibility for you killing your iPhone 3G

1. Head on over to iPhone Dev Team and thoroughly read posts entitled:
   "Redsn0w in June"
   "trois, drei, три, három!"
2. Once you have read the posts ... read them again! No seriously, if you haven't fully understood what you are doing, you may end up with an Apple shaped brick.
3. Follow the procedure for Jailbreaking your handset using Redsn0w or PwnageTool.  Redsn0w is the quick and easy method, and Pwnage requires a little more effort but is more flexible.
4. Once the phone is Jailbroken, you then need to head back on over to iPhone Dev Team and thoroughly read post entitled:
   "Ultras Now!"
5. Essentially via Cydia or Icy (Jailbreak application installers) you need to add a new repository for Ultrasn0w, and then install it.
6. Once this is completed you can then use any SIM card within a UK purchased iPhone 3G

Ok ... now we have got the basics out of the way, I guess you are here since you have upgraded your iPhone 3G to 3.0 OS and cannot get tethering or MMS working on the T-Mobile network.

Right here are the steps I took on two iPhone 3G handsets and both are now working with tethering and MMS.

1. Despite entering the correct settings, I could not for the life of me manage to get Tethering Option enabled.
2. Using Safari browser on iPhone, go to "help.benm.at" ... scroll down to "Tethering and Internet Settings"
3. Follow the link, then select UK , and obviously T-Mobile.
4. Accept and Install the settings ... BUT DON'T reboot yet.
5. The settings are actually slightly incorrect ... but what they have done is populate the fields you need, and more importantly enabled the Tethering Option.
6. Go into Settings > General > Network > Cellular Data Network >
7. Populate the fields as below:
   

   Cellular Data
   APN : general.t-mobile.uk
   Username: user
   Password: pass

   Visual Voicemail (I dont think this works but no harm in populating it)
   APN: general.t-mobile.uk
   Username: user
   Password: pass

   MMS
   APN: general.t-mobile.uk
   Username: user
   Pass: pass
   MMSC: http://mmsc.t-mobile.co.uk:8002
   MMS Proxy: 149.254.201.135:8080

8. Exit out of these settings back to your home screen and perform a hard reboot (hold down Home and Power until the iPhone Reboots)
9. Assuming you have done this correctly, you will find a "Camera" icon in the write new Message screen.  Tap the camera and choose to take a new picture or select an existing.  Beware however that taking a new picture does not save it onto your camera roll as all other camera apps do.  Might be best to take a picture first with the camera app, and then forward it as an MMS!
10. Also in Settings > General > Network ... you will notice "Internet Tethering" option.

Hope you found this quick guide helpful ... enjoy!

Addition:

For MMS it's been suggested that in Settings > Phone > "My Number" ... your number shouldn't be listed as 07xxx but starting in it's international format which is +44 7xxx etc.  It has also been suggested that anyone you wish to send an MMS to should be in international format too.  Due to International travel I always ensure that my numbers are in this format now, to save hassle.

Edit: I'll keep updating this as and when issues/changes arise.