In the old days, NFL overtime rules were simple: the first team to score a point wins.
If the Chicago Bears ran the opening kickoff back for a touchdown, as they did against the Detroit Lions on Thanksgiving Day 1980, the game was over. If the Lions won the toss, took the wind, and then let the Bears drive down the field to kick a field goal, the game was over. It was simple. Whoever scored first won the game.
At any rate, it's all worth going over again given the rule changes in recent years.
Eventually, the NFL decided that it wasn't fair that a team could take the opening kickoff, march to their opponent's 35-yard-line, kick a field goal and win without the opposing offense getting off the bench. They installed new rules for the 2010 postseason, and in 2012, they were extended to cover regular season games as well.
There's still no guarantee that both teams will get the ball in overtime. If the team receiving the kickoff scores a touchdown, the game is still over. That happened last season in the Sunday Night Football game in Week 4 -- the Saints won the toss, and Drew Brees threw an 80-yard touchdown pass on the second play to beat the Cowboys.
However, if the team that receives the opening kickoff only gets a field goal, the other team gets one possession of its own. If they score a touchdown, they win. If they don't score, they lose. If they also kick a field goal, the overtime continues in sudden-death format.
It isn't too complicated, but there are a couple interesting points: For one, any defensive score, even a safety, will always win the game. Second, an onside kickoff becomes a major weapon. In the game mentioned above, if Dallas had successfully executed an onside kick at the start of overtime, it would have counted as New Orleans losing possession. That would have meant the Cowboys would have only needed a field goal to win the game.
In the end, the only real change happens if the team that receives the opening kickoff kicks a field goal. If they do, the opposing team gets one possession to match or top them. Otherwise, it is still business as usual.
The rulebook gives the team one additional 15-minute for overtime. If the game is still tied at the end of that one, it results in a tie. They also get a two-minute warning in the extra period.
So yes, Donovan McNabb, regular season games can still end in a tie.