I got BB anywhere about 3 or 4 weeks ago now.
Yes, you are right re the mobile network. BT Mobile uses the Vodafone network.
If it interests you, here is what I have found out so far.
I got it mainly because it is a mobile phone for a fiver a month..yes you only get 50 texts & 50 call minutes for that price which I'm sure won't be adequate for many but maybe others will manage. Yes you can get more mins/texts for more money but IMO it still isn't all that competitive looking a deal.
Also, with my previous mobiles, if you went on the net you get charged for ALL data transfer. I tried not to use it but even if I opened my browser in error for a few seconds I saw 40p or thereabouts added to my next bill...very frustrating.
The BT BBanywhere gives you up to 10MB data transfer a month included in your �5 (using GPRS) so if you use it moderately you shouldn't go over, and you won't see these little extra amounts on your bills.
The other thing they push is the BT Openzone and BT Fon wi-fi access when out and about.
Also included in your �5 is 500 minutes a month access to BTO hotspots and unlimited surfing/emailing and access to your BT Broadband Talk phone line when connected using BT Fon.
All sounds good but my experience is that I have great difficulty finding any BTO or Fon hotspots in my area. There is supposed to be a BTO hotspot in my town (Alexandria in Scotland) and one in Dumbarton close by, but I can't see either of them.
Also, there are meant to be 7 Fon hotspots according to the (wildly inaccurate) FON webpage map, and I only managed to find ONE, and even at that it was a poor connection.
So in summary they have a long way to go before BTO and Fon is in any way useful.
Re the phones, it is the HTC S620 which is the default offering and you get it free if you order BT anywhere. It looks similar to a PDA/Blackberry type device, a wee bit broad looking, but quite slim I believe (Haven't seen one to verify), the HTC S710 is the 'upgraded' model that you pay �29.99 for if you order the cheaper Anywhere plans. I paid the money for the S710 and I quite like it.
There are some wee 'buggy' things that happen, like when you are logging in to BT Fon and input your password, it doesn't recognise the first character, which can be a pain if you type it all in only to have to retype it all again when it rejects it first time around.
I'm hoping that this and other little foibles get ironed out in future software updates.
It appears BT seem to be painfully slow in getting things sorted out, especially when there are many people on forums constantly moaning to them about things.
I don't mean to paint a bad picture, but just let you know what I've found so far.
I'm happy I got it, even if only for the cheap mobile as I said up a bit..and you can port your present mobile number too if you want.
HTH
Former PlusNet user. Now with BT Broadband only because it doesn't cost me anything.