Let's take the "unbendable" arm trick. Supposedly, in a few years, most can do it, right?
I've never seen "unbendable" arm as a 'trick', just a mind/body state that is essential for the doing of aikido.
The term 'unbendable' itself does not correctly describe the end state that is being trained for. It is true that if someone is testing my arm using strength, then what 'they' feel is that my arm is un- bendable, but I can choose to bend it as I wish, therefore it is only 'unbendable' to them.
As for teaching it, I usually can get beginners to 'get it' within minutes. The hard part is to maintain this state under ever increasing dynamically stressfull scenarios, this does seem to take a long time to achieve.
I've not seen the Tomiki unmoveable arm demo, and have not tried it, I look forward to Mike's report back. It seems to go against everything that I (as an aikidoka) would naturally want to do, i.e. when being pushed at right angles at the wrist, I'd want to turn on the spot, not stay stationary.