D'Agostini, 23, had two goals and two assists in 40 games for Montreal this season. In 94 career games with the Canadiens, D'Agostini had 14 goals and 11 assists.
He was originally drafted by Montreal in the sixth round of the 2005 draft. The native of Sault Ste. Marie, Ontario is earning $500,000 this season, and will become a restricted free agent on July 1.
Palushaj, 20, has been playing this season with the Peoria Rivermen of the AHL. In 44 games, he has five goals and 17 assists. He was drafted by St. Louis in the second round of the 2007 draft, and has yet to make his NHL debut.
D'Agostini is expected to join the Blues in time for their game on Thursday in Dallas against the Stars.