If anyone else playing the PC version of Oblivion was unable to complete one of the Fighter's Guild quests because of a bug that was fixed in 1.1 but doesn't retroactively correct peoples' save games, I put together a mod that lets you continue the game. It changes the win conditions for the quest to "pick up a lockpick anywhere in the world" instead of "go find the dead body that never spawned in the cave and is therefore impossible to ever find."
1. Follow the instructions on this tutorial page to create a quest script, attach it to a lockpick, and save your changes as a mod.
2. Instead of the suggested Message command, enter this command instead:
SetStage FGC10Swamp 100
3. On the Oblivion launcher window, click Data Files and activate your mod.
4. Start the game.
5. Drop a lockpick and pick it up. Tada! You've finished the quest. Save your game. You can now deactivate the mod next time you play.
Amazing what a nerd is willing to do to continue playing a game...