Quote:
...would such an impact may be absorbed spread over the length of the front end which may make the airbag sensor more or less reliable?
The airbag sensor simply detects how quickly you decelerate. Where something hits, or what that something is doesn't matter, it just cares how quickly the vehicle slows down. It's not concerned with any impact force. You could attach a chunky chain to your tow hitch (and something very solid) and drive off at speed. When you get to the end of the chain you'll stop very quickly and the airbag will go off, even though you haven't hit anything!
See here;
http://auto.howstuffworks.com/airbag1.htm
Bullbars (etc) can affect airbag operation by changing how the front end deforms in an impact. The "crush cans" Seth talks about are designed to deform upon impact to absorb some of the shock and therefore slow the vehicle down more gradually. If an aftermarket bar doesn't attach properly to the vehicle then it could change the deformation characteristics of the vehicle adversely. For example, if it attaches completely rigidly (no crush cans) then when you hit something you'll stop dead (literally, probably!) rather than the front end squishing and slowing you down gradually.
This is one of the reasons why I chose the ARB bar; I know they're very stringently tested.