teewee, I will tell you straight up that deciding to commute has been one of the best decisions I have made it a long time. I was in your shoes a year ago not really knowing where to start, but I am glad I did. So I commend your even thinking about doing it. Most don't even get that far. Now to your question:
I purchased a 7.3 FX last year (May) when getting into riding for the first time. I really liked it, then I started to regret not getting a road bike because I enjoy riding so much. Most of this was due to how and where I commute. I have a headwind everyday on the way home, I like to push myself to go faster, I am able to store clothes at work, and other reasons which make getting a road bike a good decision.
My main concern when getting the first bike was would I stick with it. I didn't want to spend a bunch and not continue. I also wasn't sure about the drop bars. A little less than a year later (this February) I purchased an '08 Fuji Roubaix on clearance and I enjoy it much more than the 7.3, not because of any flaw in the 7.3 but rather I outgrew its set of features. I will continue to use the 7.3 FX for my rain bike / cargo bike.
All that said, the 7.3 is a good bike and will certainly do what you need it to do for commuting. I put a rack on the back (the rear brake is a bit tricky to get around but it is doable) as well as fenders which I would suggest if you plan at all to ride on wet roads. I wouldn't necessarily use it for trail riding unless the trails are paved or you change the slicks to more of a cyclocross style tire which can handle road and dirt. You might even try to look at some cyclocross bikes which would be the best of both worlds.
I agree with daven above that if you are fairly confident that you will enjoy riding, then get the best quality bike that you can comfortably afford. The difference in quality of shifting and the feel of the ride will make the commute that much more enjoyable after the newness has worn off and will provide more encouragement on those down days when you are not sure if you want to ride or not.
Best of luck on your decision.