One of the things I like about this sport is the sense of community. It's nice to know that there are people out there who are willing to share their experiences and give advise.
I posted a blog a few weeks ago, Triathlon -vs- Road Bike, here is a good example of the responses I received.
I enjoy your blog. I can sympathize. I recently finished my first triathlon in 15 years. I did a bunch of them a long time ago when I was running, etc. a lot, before my kids were born.
I can't post to your blog from my email at work (Information Security Department Warning!).
I haven't any experience with a tri bike. My advice would be simply on the practical side. You know yourself better than anyone, so decide for yourself. Do you think you will stick with triathlon for a while? Or how about this check: do you have a closet full of dusty golf clubs, snow skis, tennis raquets, windsurfers, mountain bikes, kayaks, rock climbing ropes, camping gear, etc. ? If so, then get a road bike. It's more likely to be used for training, triathlon races, and later general riding.
Ask enough questions at a couple of bike shops on what size frame they'd recommend. Then go to ebay. Find a "normal" 3 to 5 year old bike, that is the right size frame, and is being sold by someone local enough to you so you can drive over and look at it before the last day of the auction. This takes a bit of looking, and is potentially disappointing, but you'll learn a lot and save 30 to 60%. Do enough homework to not overpay. Find several similar bikes (regardless of location) and put them on your watch list so you'll see their final sale price.
Once you get the bike, go to your local bike shop and tell them you want to pay for them to check it out, lube it, true the wheels and fit you (adjust the seat and bar height, maybe change the handlebar stem length) ($100 - $200). Get new tires if there's any sign of dry rot (not unusual on a bike someone's selling because they haven't ridden it in two years). This is inexpensive insurance against the hassle of dealing with a flat -- no guarantee, but it improves your odds.
Get the bike. Train, train, train. Change out the saddle, or computer, or pedals, or wheels. Train, train, train. Ride it in your first triathlon. Ride it for a year. If you're still enjoying triathlon, by then you'll be drooling over getting the latest Cervelo, or Trek, or Felt, or QR. You'll know more then what you want. You'll be able to sell your first bike on ebay, or to someone at the gym, for close to what you paid for it.
This is what I did. I still have my 20 year old Italian steel frame bike with shifters on the down tube. I still enjoy riding it sometimes. I'm also enjoying racing my lighter weight $2500 titanium Litespeed, which I got on ebay for $1000. We'll see if I upgrade next year. Those pure tri bikes are very sexy.
Also, The training is going well. I still can't get myself up early enough to start running in the morning, maybe this week!