Now, thanks to Darksteel, Fireball is back.
Change of mana cost
Back in 1993, Fireball cost 
, with an additional cost specified in its text box to pay for any additional char-broil targets beyond the first. Since the Beatdown Box Set, Fireball's mana cost had been Y: the only Y in Magic outside of The Ultimate Nightmare. The Y indicated the number of additional targets among which to distribute the damage.
Once Darksteel becomes legal in tournaments on February 20, 2004, Fireball's official Oracle mana cost will change back, and its rules text will change. It has returned to the traditional mana cost, and the additional cost for targets is again specified in the card's rules text -- but it cooks, bakes and broils just the same as it always has.
Change of rarity
In addition, Fireball is uncommon in Darksteel, so that it doesn't ruin draft and sealed deck play for Mirrodin block. Remember, despite being an enormous rolling boulder of flame, Fireball is a remarkably efficient and flexible tool for its cost. R&D considered it wise if draft and sealed decks didn't contain too many of them!
Change of art
Lastly, as you can see above (or maybe you've seen it already, if you were lucky enough to open it in your packs at the Darksteel prerelease), Fireball's art has changed for the first time in history. Check it out.
Have fun throwing balls of fire with the release of Darksteel!