The Lakers lost in the Finals to Detroit in 2004 before blowing up their dynasty by trading Shaq for Lamar Odom, Caron Butler and Brian Grant's hideous contract. They didn't return to contention until 2008 when they lost to the Celtics. Along the way, they had many misfires (Rudy T., Smush Parker, Brian Cook, etc.) before their much maligned GM's few smart moves (holding onto Andrew Bynum, accepting the gift of Pau Gasol) paid off.
This is the quickest turnaround that I can think of. The Spurs are still in the midst of their remarkable run. The Celtics mired through over a decade of futility. The Cavs and Magic are contenders for the first time in a while. I can't remember if the Nuggets were ever a legit contender. If you put Utah on the list, it's been a long time since the days of Stockton and Malone.
Other contenders from early in the decade have fallen on hard times. Nets, Sixers, anyone?
The Heat are in the midst of what could be a quick rebuild. They won the title in 2006 so you can say they were still a contender in 2007. If they score in free agency this off-season, they could be contenders in 2011. A four season turnaround.
The Pistons were last a contender in 2008. So, the question is: Is it fair to expect to compete until at least the '11-'12 season, two years away?