Is my iPhone 3GS network locked?

I have an iPhone 3GS which was supplied by Vodafone UK. I haven't used it for ages, but recently decided to update it to iOS5, and then put an O2 sim in it.

The iPhone has just had a 'Restore' done in iTunes. When I turn it on (with the O2 sim in it) it asks which language to use etc., and then attempts to Activate over my wireless connection. This message is then displayed:

If I try to activate it in iTunes, a similar message appears: 'The SIM card inserted in this iPhone does not appear to be supported.'

My question: does this message means that the iPhone is networked locked to Vodafone, or does it mean something else?