The shield is probably your best bet.
You are right about the drivers for your existing interface. I have no idea what the driver requirements are... If there are open source Linux drivers for the device, it might be possible (but probably difficult) to port the drivers to the Arduino. If the drivers, interface protocol, and internal workings, of the ENTTEC device are proprietary to ENTTEC, it might be virtually impossible to write a driver.
The Arduno should work great if you want to have a self-contained program with a limited user interface, or if you want to run a music-activated program, etc. (I've got some music-activated effects that are not DMX controlled, but with an Arduino they could be.)
On the other hand, if you wanted the kind of flexable user-interface and programming you get with a computer or a DMX control console, say to control the lighting for a play, It's probably not worth building your own controller.
P.S.
Being kinda new, bulding one still seems a little bit much at this point.
Some thoughts...
Do you have any programming experience? You will have to program the Arduino, and it probably won't be fun learning programming and electronics at the same time, and it might take you "forever" to finish your project. It sounds like you really want to make this thing, because you WANT IT, not just because you want to play around with the Arduino and learn something... In that case, BUY the shield!
If you already know some programming and you REALLY want to learn electronics, a DMX shield is a fairly simple 1st project. (I would recommend that you buy a pre-make blank PC board, or build it on perfboard... I wouldn't attemp to fabricate the PCB.) But as a 1st project with no electronics experience, there is a good chance of failure. I had my share of failures when got my 1st soldering iron and started building kits... I now have an electronics degree. 
If you had a semester of electronics under your belt, and access to a multimeter, your odds of success would be much higher. (Although you it's a bit beyond what you would learn in your 1st semester). Formal education is NOT requred! I'm sure there are plenty of people here on the forum that have never taken a programming or electronics class, but there is quite a bit to learn (depending on what you are trying to do) and I just want to give you some perspective...
