Hmm, missed that one. That really is interesting.
Ok, first off a "untrained" skill is any skill in which you do not have at least one rank. If you have not bought any ranks in climb then climb is an "untrained" skill for you. Some skills are marked with the descriptor "Trained Only". Those skills you cannot use at all unless you have at least one rank (making it a trained skill for you). You cannot atempt to Decipher Script without at least some tranning (note that some Trained Only skills, such as Sleight of hand, allow an untrained person to make a attempt for some limited uses of the skill. Those skills have a special notation in their entry, if there is no entry for an Untrained use then you cannot use it at all). All this is noted on p 83 under the Skill Descriptions heading.
So, with that in mind it seems to me that the rule for untrained skills is that, if the skill is one you can used untrained then you make a check normally and if your check is higher than 10 you treat it as a 10, but if the skill is Trained Only then you still cannot use it at all (unless the skill description specifically says you can).
Example: Bob the barbarian has 0 ranks in climb but an 18 Str, he has 0 ranks in Decipher Script (trained only) and a 11 Int, he also has 0 ranks in Sleight (trained only) and a 12 Dex.
Bob wants to make a climb check. He rolls a 13 and adds his Str mod for a total of 18, but he has no tranning so the result is treated as though it were a 10.
Next Bob wants to attempt to read some ancient runes but Decipher Script is a Trained Only skill so Bob cannot even make the attempt (note that he does not fail the check, so he draws no false information from the runes, he never even gets to attempt the check in the first place)
Finally Bob wants to palm a coin sized object. Even though he has no ranks in Sleight the skill description specifically allows him to attempt this. He rolls an 11 and adds his Dex mod for a total of 12, but again this result is treated as a 10 which is still enough to allow him to succede.
Or at least thats the way I understand the rules.