1. If all the carbon (oil, gas, coal, wood etc) burnt in one year by
all human beings were laid in a layer on the whole surface of the
Earth (including the seas) how thick would the layer be?
2. If the whole Earth were covered in grass how much taller would the
grass have to be to incorporate this carbon.(if grass is difficult use
something else eg trees, or wheat)

Please show as much data as possible.

Mean radius of the Earth = 6371 km
<http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html>. 
Using this to calculate the Earth's surface area yields  A = 4*pi*r^2
= 5.10*10^14 m^2

The annual carbon emissions in 2002 due to fossil fuel burning = 6.73
GT Carbon/year (1 GT = 10^15 grams).  If cement production is
included, then the annual anthropogenic emissions = 6.975 GT C/year. 
Neither of these figures include carbon emissions (or removals) due to
land use changes (e.g., forest clearing).  Data from
<http://cdiac.esd.ornl.gov/trends/emis/tre_glob.htm>

Density of carbon (graphite) = 2.267 gm/cm^3
<http://www.webelements.com/webelements/elements/text/C/phys.html>

If all the carbon emitted by fossil fuel burning in 2002 were
converted to a single block of graphite, that block would have a
volume of (6.73*10^15 gm)/(2.267 gm/cm^3) = 2.97*10^15 cm^3.  That's
the volume of a cube that's about 1.44 km (about 0.9 miles) on a side.

If that volume were spread out over the *entire* surface of the Earth,
it would form a layer (2.97*10^15 cm^3)/(5.10*10^14 m^2) = 5.82*10^-6
meters (i.e., 5.82 micrometers) thick.  For comparison, a human hair
is 100-200 micrometers in diameter, so the layer would be 20 to 40
times thinner than a hair.

Another interesting number is how many grams of carbon (graphite) per
unit area this thickness corresponds to.  That's simply given by
(6.73*10^15 gm)/(5.1*10^14 km^2) = 13.2 gm/m^2, or 1.32*10^5
gm/hectare (1 hectare = 10^4 m^2).

The mean carbon content of the vegetation on a temperate grassland =
7*10^6 gm carbon/hectare
<http://www.iitap.iastate.edu/gccourse/chem/carbon/images/carboncontent1.gif>.
 If the entire Earth were covered with grasslands (including the
oceans, polar regions, etc.), there would be (7*10^6 gm
carbon/hectare)*( 5.1*10^14 km^2) = 3.57*10^17 grams of carbon tied up
in the vegetation.  The ratio of the carbon emitted by fossil fuel
burning in 2002 to the total amount of carbon in this hypothetical
"grassy Earth" would be (6.73*10^15 gm)/(3.57*10^17 gm) = 0.019, or
1.9% percent of the total.  This implies that for the (admittedly
unrealistic) case you've posed, the vegetation would have to get about
2% longer to soak up all the carbon emitted in a single year.