OK this thread is again an attempt to bring together all the ideas on that illusive restaurant taste.
To my mind this can be one of 4 things:
1) An ingredient or specific preperaton of an ingredient
2) A taste generated by the cooking process of a BIR e.g. cooking 20 litres of sauce for 4 days on end
3) The fundamental difference between going to a BIR and having one at home
4) Technique
1) First off I have to say I agree with Pete, Ive been in a number of kitchens and there isn't a secret spice or ingredient that creates this taste.
One thing to bear in mind is BIR's use a lot of oil, I had a chilli masala tonight from my local and it was swimming, probably at least 3 TBSP of oil visible.
Other secret ingredients I have read about are garlic powder, tomato ketchup and a spoon of tandoori marinade in a dish towrad the end of the cooking process. I think there may be something in garlic powder but not the rest (haven't tried ketchup)
The other day I cooked an authentic curry and it had a taste similar to the elusive BIR taste, the only ingredient I could see that may have caused this was the initial fried crushed garlic. The recipe was for ~3 people and started with 8 plump cloves of crushed garlic.
2) The bottom line is the BIR kitchen is creating base sauce for a lot of dishes and a lot keep the sauce for ~4 days. This will be hard to replicate unless we all take it in turns to come round to one another's houses for curry!
3) The most depressing possibility

. When we go for a curry we take a walk through the fresh air full of anticipation, our senses aren't dulled by an hour long cooking process. Of course its going to taste great!
I will be testing this in future by getting my missus to cook a curry from my base sauce (with strict instructions of course) and I will see if it is any different having not cooked it yourself (I doubt this is it)
4) The one the BIR's would like you to believe (when they aren't bullshitting about a secret spice).
Obviously a lot of BIR chefs are very adept at cooking curries, does this make a difference?
I will do a post in the near future about my curry cooking technique but again I doubt this is really it.
If I had to point out one thing that I thought was the root of the taste I would say it is related to garlic in some way.