Sean - my truck was doing the same thing a while back but only when it had been parked overnight. for me the fix was replacing the slave cylinder because it was leaking. the general rule as I understand (correct me if I'm wrong) is to replace the clutch MC, flex hose, and SC at the same time.
Rob - check your clutch fluid level. top off and bleed if real low. as always check for leaks.
Insert pithy quote here.