Although I am a inconditional admiror of Gadm, I need to say that data on Brazil is severely out of date. It does not seem to be wronge, just not up dated for a decade. Levels 0 and 1 are fine. But take for instance level 2, of municipalities. File says that Brazil has 5504 cities and it was accurate for 1997. Today, Brazil already has 5566 cities.
As for the level 3, of municipal districts, I can not say in detail. Of course the city-grouping variable is just as wronge as the level 2 file, but I do not know if the more than 10 thousand districts have changed a lot since the version of file from gadm.
While I think it would be hard to find up to date shape data on level 3 (but not impossible trough IBGE), it is probably fairly easy. I myself have data on level 2 updated for 2006 if you guys want.
Anyway, as Brasil has done a full census in 2010, I think it would be the perfect solution to contact IBGE looking for the new shapes and data for levels 2 and 3 regarding the census.
Please please, update Brazilian data as it seems to be one of the most outdated!
My best regards,