I have to say that in my experience, libertarians are "on our side" until they have to vote. And then they vote Republican, because anti-facist, anti-militarist, anti-interventionist, and pro-choice sentiments always, always lose to the promise of lower taxes and less government.
Maybe I know the wrong libertarians, but of the tens I know only one has ever bucked the trend the other way.
[WS:] I have a rather limited experience in that area, but my limited observations suggest that at least some of them are not that much attracted by the Repug policies but rather repulsed by the "welfare state" which they associate with the Democrats. And given the forced choice in the binary US politics, they end up in the Repug camp. I suspect that in a parliamentary PR system, the libertarian party would behave more or less as Andie says - siding with the left on civil liberties, anti-militarism, and even some social policies.
Wojtek