This is my preferred system. To me, you don't want to absolute force people to vote. But you do want to remove the bias that exists because turning out to vote is easier for some people than others (and it tends to be easier for the more privileged). Other factors help too: e.g. polling is on the weekend.
Re: apathy, the solution is not to ignore the disenfranchised, but to engage them. True representation is not about suiting those with the most free time and energy.
Because they had to 'vote' (maybe "because they had to voice an opinion one way or another") people seemed to be more likely to take an interest in politics and actually learn about what was actually going on.
That's the way I see it, at least: people in Australia seemed to be more knowledgeable about politics than people seem to be here in Canada at least.
Instead, require employers to give their employees 3 hours off during the day, or something like that.
O Canada! http://www.elections.ca/content.aspx?section=vot&dir=faq...
Optional voting is a feature, not a bug. It ensures that the people who care most wield the most influence. If they collectively take the rest of the (apathetic) population astray, more people will start to care and swing the country in a different direction. There's a term for this kind of self-correcting system, but it escapes me right now.
Regardless, changing to mandatory voting would do absolutely nothing to solve voter apathy. It could theoretically have the opposite effect, as now the people who care deeply have less influence because their votes are watered down by people who only vote to avoid a fine.