EDIT
Apparently Autodesk Inventor can do the conversions for you natively. http://docs.autodesk.com/INVPRO/2010/ENU/Autodesk%20Inventor%202010%20Help/index.html?url=WS1a9193826455f5ffba22e1113c6c30c0e-3b1a.htm,topicNumber=d0e14826
Open the .stp and save as .dwg? Also, it appears that this is the application that everyone references as being able to do it (At least going from .dwg to .stp). However, it also appears that .stp is a 3D file type while .dwg is a 2d file type. To me, it would seem that the .stp file type would contain data that the .dwg file couldn't handle since 3d is inherently more complex than 2d (unless these .stp files contain only 2d images?). This could explain why there aren't many articles on converting .stp to .dwg.
I've never used the Autodesk Inventor SDK and I'm not that great at VBA either, but there seems to be some pretty good documentation on it here: http://usa.autodesk.com/adsk/servlet/index?siteID=123112&id=1079044
Here are some of the resources I found on this page:
Intro Files / Inventor VBA(.ivb) Sample file:
VBA & Inventor API Introduction (zip
- 547Kb)
The following resource will help you
get started with programming Inventor.
It assumes familiarity with Autodesk
Inventor and general programming
concepts.
Video Tutorial:
DevTV: Introduction to Inventor
Programming A self-paced video
tutorial demonstrating how to get
started developing with Autodesk
Inventor. View online | Download
Alternatively, if your only goal is to simply convert a .stp/.step file to .dwg. You may be better off finding some free converter over interfacing with Autodesk.