EDIT: I actually like Belgium's model - you don't have to vote, but you do need to show up at the polls and register attendance. You can abstain if you'd like.
Plus there's the whole freedom thing. We should keep the list of "things the government forces you to to do" as small as possible.
If anything, I'd prefer a fair voting test, where the person pulling the lever has to have _some_ kind of background knowledge.
The tests were predominant in the South and were designed so that nonwhites would fail them.
Here is a sample: http://www.crmvet.org/info/la-littest2.pdf The instructions alone made my head spin.
More from PBS: http://www.pbs.org/wnet/jimcrow/voting_literacy.html