Line Out Docks - plug 'n play portable audiophile
Line Out Docks (LOD) are typical 30-in connectors which snap into the bottom of an iPod or iDevice to grab the line-level audio information without a hint of the headphone amplifier. They can be big or small, and all speaker docks make use of LODs in one form or another.
A few good companies are:
ALO - Audio Line Out who make the excellent ALO Rx headphone amp
Twisted Cables (no website yet) - Brainchild of Headfier Qusp (Jeremey Glover - Email), fastidious and new cable maker
Qables - From the Netherlands, home of the excellent portable headphone amp, iCube