As has been noted, keepers do sometimes come up to act as an attacker during the game, though they are never really subbed out for another attacking player. A couple things to keep in mind from The Laws of the Game:
Page 15: "A match is played by two teams, each consisting of not more than eleven
players, one of whom is the goalkeeper"
Page 16: "Any of the other players may change places with the goalkeeper, provided that:
• the referee is informed before the change is made
• the change is made during a stoppage in the match"
Page 19: "• Each goalkeeper must wear colours that distinguish him from the other
players, the referee and the assistant referees"
So if you did substitute off a goalkeeper, the sub would have to have a different shirt, or if you swapped a field player with the goalkeeper, they would have to change jerseys. I have seen this happen when the goalkeeper is injured and the team has no more substitutes left - they'll put a midfielder or attacker (sometimes, but rarely, a defender) into goal, but the midfielder will play as a goalkeeper, not as an attacking player.
That being said, the goalkeeper is almost never subbed out except for another goalkeeper. The cost is too high.
EDIT: More often I've seen this happen when the goalkeeper gets a red card and the team has no more subs left. If the goalkeeper interferes with a player trying to score, he can get a red card and if the team has no more subs, they'll have to put a player already on the field into the goal, in which case he'll have to get a different shirt.