I have not yet taken Contract Law (I still have about 1 month of freedom before I am launched into the pits of Law School hell), but I did learn some information about contracts when I took a Sports Law class while completing my undergraduate degree. I learned that there is such a thing as a verbal contract, but that it is much better to get everything in writing. I would amend that statement and suggest that if you want to be in this industry, you should get all deals in writing, and never accept a verbal agreement.
There have been successful verbal agreements in the past between an agent/client, agent/team, etc, but there are too many instances where those agreements have turned to waste. Jody Vance of SLAM! Sports looks back to the deal that Wayne Gretzky originally verbally agreed to with representative Michael Barnett of IMG [Never quite a done deal]. That ended up working out just fine, but the story goes on to reveal a recent case, where a hand shake was not good enough to keep promises.
The Edmonton Oilers believed that they were going to sign free agent Michael Nylander because his agent, Mike Gillis, said that the terms of the agreement were acceptable. The problem was that the Oilers assumed that they had Nylander when Nylander never actually signed a contract. The same situation can occur in a player/agent relationship. An agent may believe that he/she has locked up a client, but until the standard representation agreement has been signed and a copy has been obtained by both parties, there is really no principal/agent relationship.
Moral of story: make sure to get everything in writing!