I spend some time stalking the ukbusinessforum, and this topic (and F/B advertising) have been discussed there. I think you've pretty much summed up the general feeling on this one. The fact that people are trying to sell it to you should be a warning. I'm always suspicious of people trying to cold-sell services like this. If it really was a hot product, people would be talking about it and beating a path to the salesman's door.
It's like the so called publications which distribute to bikers... so many thousands (or tens/hundreds of thousands) of readers... well sorry buddy, but I've never heard of it and never been handed one at one of the shows you claim it gets handed out at!
Luckily, Nik was being pretty firm with the sales woman (who, interestingly, claimed herself that she doesn't use Facebook at all so how she's qualified to sell such a service, I've no idea!) and wasn't talked into it.
Interestingly, she had made a claim that "1 in 4 people have bought through Facebook"
1 in 4 of which people I wonder! That's a bollocks claim if ever I heard one LOL
The only way you could really do it would be to have your entire site run in an iframe in 'compact' mode.
Facebook apps support pulling in of html from an external server, but becasue of the ssl and certificates and all that stuff you'll find it easier to run in a frame.
BEFORE you like the page this is what you see, after you like it, you see the normal flow of conversation on the wall. You can see it again by clicking 'welcome' on the left with the world icon.
This data is generated based on the Angry Dinosaur shops 'featured' items. The link just goes to the shop itself. I suppose this could be extended to create something that runs in the page, something that keeps it inside facebook. With the correct blue theme, customers may even be convinced its part of the overall facebook experience.