A while ago I took advantage of the offer at http://www.3dongle4free.co.uk/ and got myself a free dongle. I then unlocked it at everything wasy rosy in the world. I could put in a 3 sim of an o2 sim or an orange sim and it all worked.
I have not used my 3 sim for a while and now it will not regsiter on the network. (All the other sims I have are regsitering. (I might not have the correct apn's etc but I think that should only stop it connecting not registering. It just tells me there is a limited service))
This particular 3 sim will register if I put it into an unlocked mobile or another 3 broadband dongle.