In brewing science texts on water/mash adjustments, there are a bunch of negatives often brought up relating to carbonates in brewing liquor and very few positives.That's theoretical from my perspective (although the negatives are actual, observed and tested). If not dissolved properly it will have very little effect on pH anyway so you just have mash with chalk in it (pH will drop but not as effectively as it will if properly dissolved).
When I have used CaCO3, I've not been a fan of what it seems to bring to the palate (personal experience).
It also seems counter intuitive to me to add things like calcium which drop pH then add carbonate to bump it back up (and it needs to be dissolved in acid which also seems counter-intuitive but my chemistry is probably out in that regard). I'm able to adjust my pH without using it so I prefer to take that route.
I think dark beers will cope with chalk (most other beers won't) but I'm not convinced they are significantly improved by it.
I think braukaiser has an interesting article on adding chalk to mash and dissolving in carbon dioxide so I'll see if I can hunt it up. I think there might be a section in brun water as well. (or at least an online discussion of it somewhere recently)
I need to do some more home testing to really ascertain if my dislike of chalk is really justified but I'll get to that.
http://braukaiser.com/wiki/index.php?title=Building_brewing_water_with_dissolved_chalk