UPDATE @ 4:45pm- According to Bruce Levine and other sources the Cubs will look to add a top notch defensive outfielder via waivers (rules listed below). I am also hearing that the Cubs may put Aaron Miles, Milton Bradley, Mike Fontenot, Jeff Baker, Aaron Heilman, and others on waivers to gauge interest by other clubs and maybe to just pass off the remaining balance on their contracts to the acquiring team. especially Aaron Miles who has yet to contribute anything worth mentioning this year.
Thanks to Yahoo! Answers for this information on Waiver Rules-
The only players that can be traded are those on waivers..here is how waivers work in the MLB.
Any player under contract may be placed on waivers at any time. If a player is waived, any team may claim him. If more than one team claims the player from waivers, the team with the weakest record in the player's league gets preference. If no team in the player's league claims him, the claiming team with the weakest record in the other league gets preference. In the first month of the season, preference is determined using the previous year's standings.
If a team claims a player off waivers and has the viable claim as described above, his current team (the "waiving team") may choose one of the following options:
1. Arrange a trade with the claiming team for that player within two business days of the claim.
2. Rescind the request and keep the player on its major league roster, effectively canceling the waiver.
3. Do nothing and allow the claiming team to (1) assume the player's existing contract, (2) pay the waiving team a waiver fee, and (3) place the player on its active major league roster.
If a player is claimed and the waiving team exercises its rescission option, the waiving team may not use the option again for that player in that season. If no team claims a player from waivers in three business days, the player has cleared waivers and may be assigned to a minor league team, traded, or released outright.
The waiver "wire" is a secret within the personnel of the Major League Baseball clubs; no announcement of a waiver is made until a transaction actually occurs. Many players are often quietly waived during the August "waiver-required" trading period to gauge trade interest in a particular player. Usually, when the player is claimed, the waiving team will rescind the waiver to avoid losing the player unless a trade can be worked out with the claiming team.
So you see that the august waiver trade period is the only way guys get traded after the deadline because its a different type of trade.
Thanks to the Sandman!