Can someone explain to me the purpose of the tiny hole on the right hand of the lower receiver in which you can see a portion of the pivot detent spring? For the life of me I cannot think of a good reason for it being there!

Had to looked up the Army's Technical Manual 9-1005-319-23&P since I don't have a AR in front of me.  Photos only show a enclosed pivot pin detent housing.  Under chapter 2-17 Lower reciever and buttstock assembly Section V Paragragh 6 states.

Be sure to hold cupped hand in front of pivot pin detent and helical spring to prevent loss of pivot pin detent and helical spring.

NOTE: IF HELICAL SPRING WILL NOT COME OUT, USE A WIRE TO PULL IT OUT.

Under Chapter 3-13 Reassembly of lower reciever in shows that you need a fabricated pivot pin installation tool to reinstall the helical spring and pivot pin detent.

My guess is that the manufactuer of your reciever put that hole there for you to depress the pivot pin detent and spring and insert a bent paper clip to hold it back when installing the pivot pin.  This would negetate the purchase of a special pivot pin installion tool.
My best .02 guess.

De Oppresso Liber

I think he is refering to the fact that some mfg have cross drilled a vent hole for the front takedown detent spring hole.

This is present on some models, but not all.

IIRC the idea was that water/mud could cause the detent/spring to corrode or bind. This added hole would allow a penetrating oil to be inserted and weapon stored muzzle down. Also allows for application of compressed air if available.

Ahhhh!  That sheds some light on the subject!  Seems like it offers another avenue for crud to get in...I was going to seal it with a daub of epoxy, but I guess I'll leave it.
Thanks for the replies, guys.

I believe it is called a "weep hole".. Helps eliminate excess CLP..Some earlier manf.lowers do not have these holes.. My pre 93 Oly doesnt but my 2002 Bushmaster does as well as most other late mfg that I have seen.... John