uzagi_akimbo said:
Of course that rule absolute has just received a massive dent by the wonky Warlock class from CA, which gains the ability "Deceive item" at 4th level which allows the Warlock to do just that.
I don't see it as that big of a dent, actually. Also, the ability to Take 10 on a UMD check also appears in the Eberron campaign, as a high-level feature of the Artificer class.
In fact, the two abilities can be summarized as follows:
Skill Mastery
1. When you are not under stress, you may Take 10 on the following skills: [List of basically all of them]
2. When you are under stress, you cannot Take 10 on the above skills.
3. With Skill Mastery, you can.
UMD
1. You cannot ever Take 10 on a UMD check.
2. With [Insert Ability Name], you can.
EDIT TO ADD:
Quick aside question: Do the Warlock / Artificer Take 10 on UMD abilities allow them to Take 10 under any circumstance?
Or do they merely add UMD back to the list of skills on which one could Take 10?
If it's the latter (and I believe it is), then combining the two would allow you to Take 10 on a UMD check while under stress.