I used to live in Dallas. What sort of scene are you after? What area are you going to be in?
Bar districts are basically in four places - deep ellum, the west end, greenville ave and knox/henderson. All are a bit on the posh side (that's the nature of Dallas)... deep ellum's probably the least posh, but not by much these days. It used to be an artist community, but got basically bought up by developers and turned into more of the same shite. The west end is pretty much a tourist destination. Greenville's pretty laid back. Knox/Henderson isn't worthwhile unless you're out to tag some SMU chicks.
I grew up in Arlington, half way bewteen Dallas and Ft Worth. Back then Deep Ellum was the hot spot for the under 25 crowd. Lots of great clubs, bars, tattoo shops and so on. That has mostly died out though there are still some good places there to catch a rock show. Ft Worth is actually becoming more popular for night life than Dallas. It is only 30 miles from Dallas. City Streets is a good club there, actually 4 clubs in one building. Take a trip to Reunion Tower, the big lighted ball that you see in all the Dallas skyline pictures. Nice view at the top.