The fact that BT is using open source is hardly a surprise – after all, there probably isn't a single major company these days that doesn't have it running in some corporate nook or cranny. So the announcement of BT's open source site is significant more for the fact that it shows the company wants to be seen to be using open source.
There's not much there at the moment, and what there is hard to find because of the use of BT's TiddlyWiki:
TiddlyWiki is a single html file containing all the characteristics of a wiki - all the content, all the functionality (including editing, saving, tagging and searching) and the style sheet. Because it's a single file, it's very portable - you can email it, put it on a web server or share it via a USB stick.
What I did learn from the site despite this lack of manifest structure was that the Managing Director of BT Design and an outspoken proponent of open source and open standards is J P Rangaswami, whose blog, Confused of Calcutta, is well worth reading.
There's also an interesting list of open source projects that BT is involved: pity I can't point you to it....