I come from the camp that tends to leave it in the primary for longer and skip the secondary altogether, but I would say leave it in the primary for at least another week if it is still showing signs of fermentation. I've never had carbonation problems.
I guess I should also throw in the typical question, have you had consistent hydrometer readings for at least 3 days?
Keg #1: Oatmeal Stout (WLP004)
Keg #2: Oatmeal Stout (WLP037)
Keg #3: Denny Conn's Bourbon Vanilla Porter
Keg #4: Empty
Bottled: Apfelwein, Scottish 60/-
On Deck: Simtennial Pale Ale