According to Obsidian CEO Feargus Urquhart, we nearly got to play as a supermutant in Fallout: New Vegas. Alas, it wasn't meant to be and we've still never gotten to play as a supermutant in any modern Fallout game. Thank goodness for modders, because this new Fallout 3 mod, Metamorphosis lets you become a supermutant during your adventures in Washington DC.
The work of modder Genin32, Metamorphosis allows you to go on a lore-friendly quest that turns your lone wanderer into a big green baddie when visiting Vault 87. In the vanilla game, you discover that the Vault Tech approved social experiment for Vault 87 involved using the Forced Evolutionary Virus (FEV) to "evolve" humans. I don't think that supermutants were what the overseer had in mind, but I guess they were so preoccupied with whether or not they could, they didn’t stop to think if they should.
In Metomorphosis, instead of freeing the intelligent supermutant named Fawkes (the best companion) from imprisonment in Vault 87, you'll have the option of becoming a supermutant yourself. Although you have to leave Fawkes behind, the mod adds a new supermutant companion named Ghunorn who can be found outside Germantown Police HQ.
As a newly-minted supermutant, you'll be immune to radiation and have four new perks available: Steady Arm, Psychotic, Centaur Affinity, and Juggernaut. You'll also find that other supermutants are no longer hostile but humans are.