Derrek Lee is a 10-5 player, which means that he has been in the league for 10 seasons and played on the same team for five years, so any trade that he is involved in he has to approve. He has the right to veto any trade, but Dave O'Brien believes he would waive the no-trade option. He also believes the deal is likely to get done if the Braves are confident in Lee's back holding up through the stretch run, and the Cubs will not require a top pitching prospect in order for the Braves to acquire Lee.
If it happens, Braves wouldn't be giving up any top pitching prospect. Cubs want to shed his salary since he's free agent at end of yr.
As long as the #Braves believe Lee's back will hold up for rest of season, seems like this deal is likely to get done.
These quotes are all taken from the AJC's Braves twitter account, which is written by beat writer Dave O'Brien.
Lee is batting .251/.335/.416 (batting average, on base percentage, slugging percentage) with 16 HR. Derek has won three gold gloves (not that they are a good measurement of defense) at first base and would be an obvious upgrade all around over what Troy Glaus has done over the past few months.
Lee is in the final season of his contract and is owed around $3.25 million for the rest of the season.
Martin Gandy of Talking Chop has an in-depth review with quotes and updates about the trade at Talking Chop.