Thanks for your feedback Achim
Now I did:
v.centroids --overwrite input=teste01@Brazil output=teste01_centroid@Brazil cat=1 step=0
Vector map <teste01_centroid> already exists and will be overwritten
Processing features…
2 new centroids placed in output map
Copying attribute table(s)…
Building topology for vector map <teste01_centroid>…
Registering primitives…
4 primitives registered
15 vertices registered
Building areas…
2 areas built
2 isles built
Attaching islands…
Attaching centroids…
Number of nodes: 4
Number of primitives: 4
Number of points: 0
Number of lines: 0
Number of boundaries: 2
Number of centroids: 2
Number of areas: 2
Number of isles: 2
v.category complete. 2 features modified.
(Fri Nov 05 14:23:15 2010) Command finished (0 sec)
and then:
v.to.rast --overwrite input=teste01_centroid@Brazil output=teste01centroid@Brazil use=val value=5
I got:
Loading data…
Pass 1 of 2:
Reading areas…
Reading features…
Writing raster map…
Pass 2 of 2:
Reading areas…
Writing raster map…
Converted areas: 2 of 2
Converted points/lines: 0 of 0
v.to.rast complete.
And now It seems all OK 
But I do have 1 (last) question:
When I use the Query pixel value feature, I get this:
568209.941176|9248338.294118||5|Value 5 Does this means that each pixel has a raster value of 5 right? Because I’m also getting Value 5 and I don’t what it is…
Thanks
Luisa