It's been a while since you posted your question. For the latest details, see: http://vnews.ironmanlive.com/vnews/howto
In short, here's how it works, there are three ways to get into the Ironman World Champtionships in Kona: 1) get a slot through the lottery; 2) get a special entry from the World Triathlon Corporation; or 3) qualify at one of several Ironman or 1/2 Ironman events.
There are 150 US and 50 international lottery slots, you have to register (I'm not sure what the cost is), and if you want to increase your odds, join the "Passport Club" for another $50+. The odds are long (rumor has it that there were about 5,000 lottery entries for the 2005 race), but I know at least two people who got in that way.
If you're a celebrity or have some other special story (Challenged Athlete, etc.) you may be able to get a slot - there are very few of these, and they are usually the subject of the special interest stories you'll see on the NBC broadcast.
Finally, there are about 1,500+ qualifying slots given out at races around the world. The list is at: http://vnews.ironmanlive.com/vnews/qualifyingraces
Each has a certain number of slots (I won't go into how many or why - you can get the information on-line). Each age group has at least one slot, some have more (races with a lot of slots, such as IM Canada can have ten or more slots in certain age groups). The day after the race, the top finshers in each age group have the right to claim their slots - if there are five slots in M35-39, the first five in the M35-39 age group have until a certain time that day to claim them - they have to show up with cash or check in US$ (something like $475 or $500). Show up a minute late or without the $'s, and you're out. Unclaimed slots go to the "roll down", where the next finishers can take the unclaimed slots. Let's say only three of the five M35-39 slots were claimed. The remaining two roll down - they start calling names in the M35-39 age group in order of finish - if your name gets called and you are there with the $'s in hand, you're in. It's happened that in some races slots roll down very far (Vineman 1/2 Ironman M40-44 in 2004 rolled to something like 100th place of 200 in the age group). By doing a little research you can find a race where your odds of qualifying may be better than average.
You are lucky, because they have added a new race that is focused on Hawaiian locals, the Honu 1/2 IM. It has something like 30 slots for Big Island locals, another 20+ for state residents and the a few more for others. Check out http://www.honuhalfironman.com/