+1 Big Kahuna. If you have never ridden before and just want to see if you'll like it, go with a hardtail. But don't get some cheapo Wal Mart bike- you'll hate it. Pick up a Gary Fisher or a Trek (maybe even a decent Specialized) for around $400 or $500...heck, look on Craigslist and buy a used one. Take it out and ride it to see if the sport's for you. If you hate it and don't go, then sell the bike for a couple hundred, and you're good to go with minimal investment.
I started riding a year and a half ago, and it's the best thing I've ever done since taking up riding standups. I took a big trip back west this year, went to some killer riding spots in Colorado, Utah and Arizona. I'm going again next year. It's a peaceful sport if you want it to be, or it can be downright crazy if you choose-it's very adaptable to your level of skill/age (LOL).
I started with a Trek hardtail I got for $430. After I got hooked, six months later I bought my full suspension. After deciding that I will be doing this for a long time to come, I just built my first full suspension bike (the KHS), and I am officially hooked. The weather is cooling down, so it's perfect to hang up the life jacket and wrestling shoes and then pick up the bike and hit the trails (no lycra, please).