Just so you all know, you actually do derive some small benefit when a city center is placed atop a resource (whether done before or after revelation of said resource). AFAIK, the 'bonus' is always capped at one resource unit though, but you will still get one extra hammer by having most strategic resources in your city center, one extra food typically by having most health resources in a city center, or one extra gold (again, typically; there may be exceptions) by having a luxury in the city center. As such, rather than the typical 2-food 1-hammer (1-commerce?) yield for your city center tile, a city founded on top of pigs for example would yield 3 food (post AH), and a city founded on top of iron would yield 2 hammers per turn.
I believe this can compound with the one extra hammer you can get from settling on plains hills (the only non-resource city-center bonus achievable that I am aware of*), so theoretically you could have a 3-hammer city center by settling a city on a strategic resource on a plains hill.
I hope that helps some; it does seem to make some sense I think (IMHO at least) to handle things that way, as otherwise the potential for having cities whose base yields were like triple a 'normal' city's would probably have been too unbalancing in the very early game.
(And
mynystry, just to be clear, you will still get benefit of any resource in a city center in the sense of your civilization using it (as a strategic resource/lux/etc., after applicable techs of course), but you just wouldn't get [all of] the bonus yield from that individual tile. Because of this, deliberately placing a city atop a crucial or rare resource (e.g. oil) can sometimes be a decent play simply for the added protection of the city's garrison (especially in the sense of adding to the chance of thwarting spy sabotage attempts).)
*Edit: aside from one extra commerce for riverside maybe now that I think about it, but really, who is that concerned about commerce in the opening anyway?
