Windows has been doing that now for quite a while, with the warning bit and all!

this guy here:

http://www.brunazo.eng.br/voto-e/indice.htm

has a full site (in Portuguese, but some of the collected texts are in English) about the problems with Brazilian e-voting systems.

Basic criticisms are: there's no real way to audit elections outcome without delivering the contents of the votes. His claim is that a simple paper trail would be enough, but the Brazilian electoral committee (TSE) refuses to do so for cost reasons and has successfully lobbied congress for many years into keeping this out electoral laws.