What do I do with receptacles behind built-ins?
Since all electrical junctions must be accessible, you have a few options.


Completely remove this section of the circuit.


To do this you will have to remove the wire feeding the receptacle, which means you will have to locate the source of the wires. The cable may come from another receptacle, or directly from the service panel. Either way you'll have to remove the receptacle, and all the wire feeding the box (I think you can leave the empty box in the wall, but you may want to remove it anyway since you no longer need it).
If the receptacle feeds another receptacle, you'll have to rewire the circuit to compensate for the removed section. 

Terminate the circuit.


Since the junction will still have to be accessible, you'll have to cut a hole in the built-ins to allow access (if they have a back).  Once that is done, remove the receptacle, cap the wires with wire nuts and tape, and install a blank cover plate on the receptacle box. If this will be inside a cabinet, check your local codes to make sure this is allowed.
In this case if the receptacle feeds another receptacle, you can just connect the wires together (junction), and close it up with a blank cover plate.

Integrate the receptacle into the built-ins


This approach is similar to terminating the circuit, and you'll have to cut a hole in the built-ins (if it has a back). Again if the receptacle will be inside a cabinet check local codes to make sure this is allowed.
This might not be the best idea in the case of shelving units, since it could easily lead to a fire or electrocution hazard.

As B Mitch points out. Moving the receptacle to the other side of the wall may also be an option.


In this case simply cut an access hole on the other side of the wall, flip the box around to the other side, and install the receptacle on the other wall.



If it was me, I would remove the receptacle completely.  I'm not so keen on having live wires just hanging out, and receptacles in enclosed spaces (cabinets for example) makes me nervous. 

I'm not sure what you mean when comparing dry/wet locations, the issue here is accessibility.  The only difference between a wet location versus a dry one, is if you integrate the receptacle into the built-in in a "wet" location (a bar for example) you would have to switch to a GFCI receptacle.