Hi Steed_and_emma , in answer to your question its pretty easy to obtain an IP for someone connected to you, however lets look a little deeper at the rest of what your asking and we can see a clearer and I hope more informative picture for you.
Firstly your ISP is not going to be involved in tracking or watching your activity, the idea is that companies paid for by the media Cartels will scan P2p networks and look for those they believe are sharing their employers copyrighted content , the normal "best" practice after this is to try to obtain a sample of the file and then contact your ISP to see who the current IP address they are concerned with is assigned to, unfortunately many of these companies who work for the Cartel are often too incompetent or too lazy to actually obtain a sample of the file instead preferring to bluff, currently most ISP companies charge for this data (about £40-50 in the UK) and then the right holder (or their "agent", usually some poor call centre worker) has the option to either ask you for some pre trail settlement (money) or take you to civil court, this costs them money again so its a last step.
The new method as outlined in the UK digital economy bill is somewhat different, instead of dealing right away with any request for the subscribers information, your ISP will be obliged to send out "naughty boy" letters that let you know the Cartel believes you are sharing their "property", the newly proposed legislation seems to indicate that you,ll get at least 2 of these letters before your subscriber information is handed over, under the new rules no charge will now be made for the data, thus we will likely see an increase in false claims.
Now where does that leave many of us in the UK you may be asking ? Well, in an awkward place for sure but there are some methods and concepts that can ensure we are all doing our best at all times to protect ourselves from these anti p2p attackers, the first one is to ensure your IP is dynamic and thus its a little harder for those trying to compare IPs to ascertain whether your the same guy/gal who they detected yesterday in their "net", the other is to ensure that you only share with those who are also sharing content as the Cartel bots do not share files, a further action is to not use the same winmx name all the time and make regular minor adjustments to things like lines types, quantity and type of file shared etc, all these tricks are pretty simple and will allow the casting of doubt on any claim, there are some other options that we at this end of the network can aid you with such as weeding out all "network scanners", those who look for folks sharing files and blocking them from the community patched network , we have held off of doing this up to now as it will certainly impact on host files users as they are the sole conduit used by the cartel to scan folks files, however if this battle hots up this will be employed along with maybe some counterflooding tricks that will originate from some of our machines and will be invisible to community patch users but will contain random IP's and junk data, This is the same method the Pirate bay have introduced to halt similar privacy invading attacks on their users and if we monitor any increase in a major way of such network scanning attacks its certainly something we will look to rushing into service.
I hope this helps you plan your activities, I always at this stage make it clear that I'm not inciting infringement, I do however believe in your rights of privacy, free association and the friendly sharing of your own creative content (of whatever sort) without some fat cat business types trying to steal or monopolise just how you do so.
Btw in answer to your question I dont believe there has been any major loss of folks in the uK this month, however its a possibility we could lose some and gain some others, its swings and roundabouts and many folks swap from one network to the other and back again when they see its not as cosy as WinMX.