Does the iPhone's passcode protect the SIM? So my iPhone was just stolen. I have a long - 8 digit - pin. I'm also on the beta version of iOS 7. I think I have Find my iPhone installed and enabled.

It's apparently "offline" - it's standard practice to turn a phone off straight after stealing it, according to the police. I gave the police my Apple logon details, and I'm hoping the crims are dumb enough to turn it back on at some point.

I don't know whether I need to or should cancel my SIM. I'm sure they won't be able to get in to the phone for a while, at least until iOS 7 is publicly cracked. My concern is them taking out the SIM and racking up a phone bill. I'm on Tesco Mobile so the contract is cheap, but overages and non-UK-geographic calls are a bitch.

Is my SIM in any way protected?
If you did not use a dedicated SIM-Pin then your SIM is not protected.

In other words, if you lose your iPhone someone can easily take the SIM card out of the iPhone and use it it in a different phone.

The iPhone PIN only protects the contents of the iPhone NOT the SIM card.
You have to use a SIM pin to protect the SIM card, it's available in the iPhone settings.

So if you lost an unlocked/unprotected SIM card, you should immediately call your provider and cancel the SIM card. They can send you a new one.