Hello Daniel,

This should help -
"The protruding gap fillers, two of thousands in use across the underside of the shuttle, are not associated with foam debris shed by Discovery's external fuel tank during launch. The stiff, ceramic cloth material is inserted between heat shield tiles to ensure smooth air flow and to prevent adjacent tiles from rubbing as the shuttle's aluminum skin flexes during launch or due to temperature swings...According to documentation of a similar incident during an earlier mission, engineers believe gap fillers typically dislodge as the shuttle goes supersonic during launch... The concern is that one or both gap fillers in question could "trip the boundary layer" during re-entry, that is, disrupt the smooth, laminar flow of supersonic air across the belly of the shuttle and create eddies of turbulence that, in turn, result in higher downstream heating."
<a href="http://www.spaceflightnow.com/shuttle/sts114/050802goforeva/".Source - Spaceflightnow is a great site for up-to-date information.

Also, Wayne Hale, NASA's deputy shuttle program manager said "Discovery can return safely without either of the gap fillers in place, Hale said. One of them keeps tiles from vibrating against each other during liftoff and has no purpose for re-entry; the other is designed to prevent repeated overheating of a gap between two tiles, but not having it in place during a single re-entry would still be "well within our safety margins..."

Regards, Vince