For the 2010 Olympics, there are two qualification heats. The top 3 in each heat advance directly to the finals. From what I think I heard while watching, the next 6 in each heat will advance to the semi-finals. The 12 (6 from each heat) that advance to the semi-finals will compete for the final 6 spots in the finals. I haven't been able to fine the rules or another site that gives me a breakdown at these Olympics, but that is my understanding of how it's going to work.

I'll answer this question in the hopes that someone else can build on it.

http://www.vancouver2010.com/olympic-snowboard/

From this site, I can see try to guess as to how qualifications work.

Snowboarders all go through the halfpipe individually and record times. They go through the halfpipe twice. There best time is then ranked among all the snowboarders and the 32 best times then go on to a round robin format.

4 Snowboarders each then race and the top 2 in each race advance on.

So in the first round, there are 32 / 4 = 8 races and 8 x 2 = 16 advance on to the next round.

Then there are 16 / 4 = 4 races and 4 * 2 = 8 advance on to the next round.

This goes on until there are 4 left in which case the top 3 get medals.

The qualification rounds are both to qualify and to determine which 'lane' you race in. If you have the highest time, you get the preference of choosing which 'lane' to race in (for instance, the inner lanes are more desirable than the outer ones)

I haven't watched the halfpipe but I am basing this answer on watching the men's snowboarding cross which I feel is similar.