Update: Fallout version 3.1—in beta as of this writing—resolves this problem (hallelujah). If you're still experiencing this problem, and if 1.3 is still in beta, you can install the beta via Steam by following these instructions.
While in Workshop mode, pressing the "F" key will rotate the object you're trying to build and place. The key for this action isn't mappable through the in-game UI.
That works fine with the default key mapping. But I always remap my movement keys to ASDF, so in Workshop mode I can't move forward. I've been site-stepping and backing up to get around town.
Is it possible to remap keys if their mappings aren't exposed through the UI? On the PC there's an intriguingly named file at