Hi Adam,
For supported networks, see
http://www.taniwha.org.uk/gprs.html as we use this list to work out the details.
For supported phones, in theory, any mobile which supports bluetooth and has a 'modem mode' (i.e. supports letting your laptop connect to the internet using it) will work. Pretty much all new Nokia, Sony Ericsson, Motorola handsets work.
We have occasional trouble with the odd handset - e.g. I found my LG U880 on UK's Three network needed an extra tweak. Often if this is the case we can work out a solution interactively with the customer. Most of the time "it just works".
Hope that helps,