Ok, my personal take (which willl likely differ from everyone here) is as follows (and is still incomplete as it will no doubt change as I see it in play - it is, of course, a mistake to commit yourself to solidly to any house rule before you get to actually see it work).
I think that 'trained is trained', but that a rogue who has been trained ought to have an advantage at thievery over a non-rogue with the same training. I'm just not yet sure what that advantage should be.
I don't like the idea of just giving the rogue a bonus to the skill.
I don't like the idea of declaring that some objects are out of reach to a non-rogue.
So I am considering changing the consequences of failure. I.e. the rogue, who has spent his life attempting these actions - both successfully and not - is better at recovering when things go wrong.
So:
Disable Trap: If a failed check would have set off a trap (missed by 5 or more), the rogue gets an immediate dexterity check to avoid actually setting it off. Other trained individuals do not.
Pick Pocket: If a failed check would have resulted in the rogue's attempt being detected, he gets an immediate bluff check (opposed by insight) to deflect suspicion. Other trained individuals do not.
I'm still not sure what the advantage to Open Locks and Sleight of Hand will be, and there may be no advantage. Or any advantage may turn out to be situational. For example, if it is part of a Skill Challenge and a failure would result from a failure in either of these skills, as appropriate there may be a way to mitigate the failure (although not turn the failure into a success).
But then again, to me restoring some flavor and individuality to the classes is more important than simply distinguishing between trained and untrained. The fact that they will succeed far more often does that all by itself, so you don't need to create an arbitrary benefit beyond that +5.
If the DC is 20 and your trained character has a +9 and your untrained character has a +4, the simple fact that the trained character will succeed twice as often is all the advantage trained needs over untrained, imho.
And all of the above subject to change if we don't like how it plays out.
Final Note: One could also take this idea (mitigating failure, rather than increasing the chance for success) and use that as a distinction between trained and untrained. Either exactly as above (those trained in thievery get a mitigation roll after a failure) or by widening or narrowing the gap between 'failure' and 'failure with consequences' for trained versus untrained characters. This gives you a significant distinction while still enabling the character to try that difficult task despite not being trained in the skill.
Carl