From Jayski.com:

Kevin Grubb Trying to Return to the Busch Series?: Kevin Grubb, who was suspended from NASCAR in March 2004 for drug use, was initially listed on this week's entry list from Nashville Superspeedway, as the driver of the #56 Mac Hill Motorsports Chevy. However, the official entry list from NASCAR had the driver listed as TBA. A NASCAR official states that Grubb has applied for reinstatement to NASCAR, but has not yet been approved.(6-5-2006)

UPDATE: Busch Series driver Kevin Grubb, who was suspended by NASCAR in March 2004 for violating the sport's substance abuse policy, has applied for reinstatement. According to a NASCAR official, Grubb has applied for reinstatement but has not been reinstated at this point. At the time of his suspension, officials said that Grubb needed to meet terms outlined by NASCAR, including counseling and periodic random testing. The driver, a native of Mechanicsville, Va., had finished 19th in the standings in 2003. Grubb was expected to run a limited schedule with Team Rensi in 2004 but had not competed in a race prior to his suspension.(NASCAR Scene)(6-6-2006)
UPDATE 2: NASCAR.com has now updated their entry list to include Kevin Grubb.(6-7-2006)
UPDATE 3: A NASCAR official has confirmed that Grubb has been reinstated.(6-7-2006)