In a FAQ, the company says: "Virgin Media WiFi uses a separate connection on your Hub to access the internet than the one used by your home broadband network. The data from your home network is completely separate from Virgin Media WiFi traffic, meaning the broadband connection you pay for is exclusively yours, and just as secure." That distinction should ensure that no-one has access to your home network or the data passing through it. Conversely, however, you won't be able to see when someone is using your router-powered public WiFi connection.
The scheme is conceptually similar to BT's FON WiFi service. Both are sold as perks for their respective customers, enabling free, stable connections when 4G just doesn't cut it. There's one key difference between the two, however: BT allows non-BT customers to connect to its WiFi network for a small fee. Virgin Media's equivalent, it seems, is only available to subscribers paying for TV, broadband or mobile phone services. To be useful, however, both companies need a large, convenient network of hotspots. The 'opt-out' approach by Virgin, then, is hardly surprising.
On a page explaining the service, Virgin Media says it will "complete the rollout" for SuperHub v3 routers later this year. It's also planning a similar switchover for Super Hub 2ac routers, though for now it's merely a "pilot." If you're a Virgin Media customer, look out for the email -- you can then log in to your account and opt-out through the My Profile section, should you distrust the company's claims of an unhampered experience. Otherwise, just sit back and download the Virgin Media WiFi app for iOS and Android -- it's all you need to start using the company's growing public network.