I looked at and tried a LOT of fanny pack gun carriers until I finally decided the Blackhawk Weapon Carry Case was the best:
http://www.blackhawk.com/product1.asp?P=60WF
This used to be known as the "5-5-10" because you can draw the gun and fire 5 shots in 5 seconds at 10 yards.
Unlike most gun packs, this one doesn't look like a gun pack. It's smaller and more compact then the usual huge gun packs, and looks like any hiker's fanny pack.
The gun compartment is in the back, and is opened with an ambidextrous "rip cord" pull cord. Inside the gun compartment the rear face is covered with loop Velcro so any holster or accessory can be attached.
The front side of the compartment has two elastic magazine carriers.
The front pocket covers the rear gun compartment and holds your wallet etc.
There are two "wing" compartments for smaller accessories.
This is an absolutely first class quality pack, made of the best materials and has a "finished" look even inside.
To actually position the gun, the pack comes with a "one size fits all" holster with a thumb break, but any makers nylon fanny pack holster will attach to the Velcro.
Above all, this pack is FAST. With a little practice you can draw nearly as fast as you can from an IWB belt holster.
Unlike the packs that close with Velcro, the nylon zippers won't open, exposing the gun, or worse allowing it to fall out, and it's quiet to open.
Of all the fanny pack gun carriers, this is the fastest, most secure, highest quality, and least "gun" looking pack I ever saw.