Smith has essentially been a free agent ever since the Lions decided not to tender him a contract offer this past offseason. The Lions decided to go in a different direction at running back after Smith rushed the ball only 34 times for 133 yards in six games in 2010. He was placed on injured reserve with a thumb injury, and this came one year after he had his season ended early by a torn ACL.
As for Davis, he has been a free agent since the Dallas Cowboys released him this past July. The Lions brought him in for a workout in August, but there were no developments following his visit to Allen Park.
In all likelihood, the Lions were probably just kicking the tires on Smith and Davis. That is usually the case for most in-season workouts. I don't anticipate either being signed, although things could potentially change if any injuries occur during the second half of the season.
UPDATE: Adam Caplan reports that 24 players worked out for the Lions this week. In addition to Smith and Davis, Antwaan Randle El, Brian Brohm and former Lion Aaron Brown were some of the guys on the list.