After trawling different forums I finally found a way to get the BBC iPlayer fully working over a 3G connection on my HD2.
To accomplish this I had to fix two problems -
1. My HD2 showing as an unsupported model on the iPlayer website
2. The iPlayer refusing to stream over my 3G connection due to it only being available to Vodafone customers (which I am)
The first problem is solved by applying a about:config tweak in Opera. Open Opera and type "about:config" (no quotes) into the address bar.
Scroll to user prefs and find the Custom User Agent field.
Delete EVERYTHING in this field and replace it with "HTC_Touch_HD_T8282" (no quotes).
Scroll down to the bottom and hit Save.
Now you have access to the iPlayer mobile website, and you can stream over wifi to your hearts content. But streaming over your phones internet will bring another error.
NOTE: The following fix has only been confirmed to work for Vodafone customers
When you put your SIM into your TouchPro2 it configures all connection settings for you. What it doesnt do is create the settings for a wap connection which is what you will need to use to connect to the internet in order for the BBC iPlayer to recognise you are on a vodafone connection.
To do this on your HD2 go to Settings --> Menu -> All Settings --> Connections --> Connections --> Add a new modem connection
Enter any name for your connection (vodawap is a good one) and choose Cellular Line (GSM) as the line.
Access Point Name: wap.vodafone.co.uk
Now according to the source of this fix you need to select the wap connection as defualt, and switch back once you are on the iPlayer site.
However, my HD2 is set to Auto Pick and works fine..
Hope this helps you as much as it did me!