The trade was announced late Tuesday.
D'Agostini had two goals, two assists and 26 penalty minutes in 40 games for the Canadiens this season, and scored 12 goals with nine assists and 16 penalty minutes in 53 games in 2008-09. He was Montreal's sixth-round draft pick in 2005.
D'Agostini is expected to join St. Louis for a game in Dallas on Thursday.
Palushaj is 20 and was the 44th overall pick in the 2007 draft. He had five goals and 17 assists in 44 games for Peoria of the American Hockey League this season.