Macgirl has some pictures and a review in
this thread, but I thought I'd add my thoughts to this one.
It's about $20 from major online retailers, and fits very snugly in a small classic Timbuk2 messenger bag. According to other folks it also fits with room to spare in the medium messenger. I'm sure it would fit in other bags from other companies, it's stiff foam but still a little malleable so there is room to squish it in on the ends if the bag is just a little too small.
In my small Timbuk2 bag, the insert itself holds:
1 tele zoom
1 wide-normal zoom
1 dSLR body (attached to one of the zooms, stored vertically)
1 flash
1 P&S or 1 prime lens
The inner pockets of the bag are a little hard to access because of the tight fit, but they are still useful for holding infrequently accessed items, like batteries, flash cards, lens caps, etc.
The insert itself works really well, everything is easily accessible and with the bag closed it looks like an old, beat up messenger bag with little clue as to what's inside. The main disadvantage of messenger or sling systems, though, is that all the weight is distributed along one shoulder and the chest, which makes it uncomfortable for carrying heavy loads for a long period of time. So I'd say that this setup is good for heavy loads for short shoots (like event shooting) or lighter loads for long shoots (like sight seeing).
Some photos:
With insert, bag closed:
Bag open (you can see the insert pokes up a little over the edges):
With bag open, L to R: tele zoom, wide-normal zoom, P&S or small prime (under the 2nd flap), flash, extra squish room.