In Unity 4.0(5) you can do this with a conversation script modification - I've posted the script in question here:
http://www.ciscounitytools.com/Conv_Mods.htm
It's not supported by TAC, but a few sites are using it - basically if you call from a known device (i.e. you call directly from a phone that's in you're subscriber record's extension list) then you wont be prompted to enter a password.
In 4.1 (coming out later this year) it'll be configurable on a per user basis - the script mod I have out there is a system wide change.
You can't do this in versions prior to 4.0(4) because the scripted conversation engine that this functionality is built on wasn't in the product yet (at least not in its final form).