Is it allowable to have an electrical cable going into the attic through a soffit from outside?
1It is very wrong. You CANNOT have a cord passing through siding, walls, or any other finished structure/surface. For both code and safety reasons.
The receptacle MUST be outside. If it is close to the heat tape you'll just need to coil the cord up.

National Electrical Code 2011
Chapter 4 Equipment for General Use
Article 400 Flexible Cords and Cables
I. General
400.8 Uses Not Permitted. Unless specifically permitted in 400.7, flexible cords and cables
shall not be used for the following:
(1) As a substitute for the fixed wiring of a structure
(2) Where run through holes in walls, structural ceilings, suspended
ceilings, dropped ceilings, or floors
(3) Where run through doorways, windows, or similar openings
(4) Where attached to building surfaces
Exception to (4): Flexible cord and cable shall be permitted to be
attached to building surfaces in accordance with the provisions of
368.56(B).
(5) Where concealed by walls, floors, or ceilings or located above
suspended or dropped ceilings
(6) Where installed in raceways, except as otherwise permitted in this
Code
(7) Where subject to physical damage