I think I'm probably being a bit stupid here, but what's the disadvantage given for using a basic skill without training. Are challenge/misfortune dice added? Or do you just not add expertise dice?

Correct.

The main difference to advanced skill is that you have to acquire those before you can even try to use them. As soon as you have acquired an advanced skill, you roll it in the same way as you would a basic skill, i.e. throw in expertise dice when you have trained the skill, a fortune die for a specialisation etc.

mcv is correct, but the GM is of course always free to throw in misfortune (or up the difficulty) if he thinks a certain task is a lot harder for the untrained.

gruntl said:

Quite a feat, considering I haven't even posted in this thread yet. (Or is this just a general acknowledgement of my correctness?)

In any case, ozean and gruntl are both correct. Training adds an expertise die. Without training, you don't get that expertise die. Basic skills can be used untrained by anyone (they are already considered to be acquired), whereas advanced skills need to be acquired first (after which they can be used untrained), and then trained (after which you get an expertise die).

And as always, the GM can always decide that certain rolls deserve extra fortune or misfortune dice for a wide variety of reasons. Not being trained in a particularly tricky use of a basic skill could be such a reason.