ZDNet UK has hit upon one major problem for how the Digital Economy Bill will handle copyright infringement. The short version of the plan is that copyright holders, can notify a provider of an alleged infringement, and the provider is then meant to match this up with the customer involved and keep a record of the number of strikes. Then at some point in the future the copyright holder can apply for the actual information that identifies the violators, the presumption being that they'll go after those with the most strikes.
With fixed line broadband this is relatively easy. Providers keep records of who was assigned what IP address from their IP pool at a specific time, and the tools the copyright holders will use will also see these IP addresses. Alas the situation is different for mobile broadband. Put simply the mobile broadband providers have a limited set of public IP addresses, and map a number of users to one of these IP addresses in a similar fashion to how a broadband router shares one public IP address between multiple computers in a home.
This means that while copyright holders will know the public IP, this will not uniquely identify the mobile broadband connection that was used. Mapping is not impossible, but it seems that on a day to day basis the mobile broadband providers do not do this (although they may have resources to do so on a small scale, e.g. for abuse and police investigations). With the millions of broadband dongles now in use the UK, creating a system to track all of these and the issue of Pay As You Go services adds to the complexity.
Of course it may be that the media industry is willing to overlook the mobile broadband market, but while it may not be used widely for copyright infringement now, if those exploiting the system become aware of this they will simply move to the mobile services, making it an issue. To some extent the data allowances on mobile broadband make it less attractive for file-sharers, but 15GB a month is an awful lot of music tracks. Lets not forget that Napster was big back in the days when most people still had dial-up. There is also the disparity between the advertising telling us that mobile broadband is fast and great, but when they want it to appear slow and less useful compared to fixed line broadband the tune changes. 3 certainly appears to have people making heavy use of their service, hence the introduction of traffic management.
Looking forward to services like LTE/4G one presumes the mobile industry will resolve this NAT like architecture, since once they introduce lower latency solutions people will try to do more things like connect games consoles and other devices that may not like the restricted access that is common at present. Some of the early satellite broadband providers had similar problems, but we assume the next generation being launched now have resolved this, though no matter how good the bandwidth is to and from a geostationary satellite very little can be done about the massive distance the data travels, which kills latency sensitive applications such as gaming.