Around this area, one of the primary forages for bass is blue gill.  There is a particular lake that I fish a lot and has been producing big bass in large numbers.  But it is also jam packed full of blue gill.  I have noticed when I fish from bank in the evening that the blue gill bite starts out pretty strong and then slacks off as the sun starts going down.  About that time the bass bite starts picking up.  I am assuming that this is because the bass are comming in shallow to feed on the blue gill and other critters that hang out in the shallows.  My question is, if the blue gill are biting is that sign that are no bass in the area?  Are they scared off by the presence of the bass or do they just become more wary and are less prone to nip at my lures?

I think you're right. The bluegill are probably close to cover or maybe down deep once the sun begins falling. The ones that remain in the same place will either not be there for long or are too busy looking out for bass to hit your lures.

I have always been able to tell when a bass is passing through if I see bluegill minnows bunch up and rush close to the bank.

I've noticed something similar on one of the local lakes that I used to wade in. Many times I would be surrounded by Bluegills and as it was getting towards dusk they would really thin out. Sometimes when I would be leaving, walking the shore back to the car, I would see a Bass come in and ****** an unlucky Gill that failed to dissappear with the rest of the school.