Most pass rushers have these kind of slumps. For instance, I remember Strahan having a long stretch from the early '99 season to late in the '00 season where he was not getting sacks.
Last time I checked, Tuck has about 20 tackles on the year. That's not bad for a defensive end. Keep in mind that he is solid against the run and often plays DT in pass rush situations, where his job is as much to draw double teams to free up a teammate as generate his own pressure. I think sacks will come eventually. My perception is that the whole thing is being overblown.
Tuck has taken a beating over the past five years and I'm certain that his body is not what it was in '08. If we are expecting him to have those all-pro type seasons every year, he may never live up to expectations. If we start looking at him as a formerly great player who is still generally solid, very versitile and savvy, an important cog when the defense is playing well, that may be more realistic. I'm sure he still has some huge games left in him, as he showed last January.