It takes persistance. That's probably the only way you'll get in with a DSLR without sneaking one in some how. Plus, there's no way I'd want to be in the middle of the floor at a Drop Kick Murphys show with $5000 worth of camera equipment.
Awww, why not?
I've shot a few shows, always with my Canon 30D and 50mm. Granted, it's not 5000$ worth of equipment, but it's not the cheapest equipment either and to me it's a huge investment. It was well worth it! The best shots come from up close and personal, and in the #$@& as they say
I'm definitely no professional, but my advice to OP is to get a fast prime lens that can handle low light. Then practice, practice, practice. I would avoid flash unless you have one you can control the output of and bounce. Direct flash in the performer's face is a pet peeve of mine. It takes away from the feeling of the live performance because it can completely wipe out all ambient light. Oh yeah, and have fun! I am still in practice, practice, practice stage, but as a fan of music it is incredibly fun practicing
As for getting into venues, I'd start small. Local bands, smaller acts,, different venues. That way you can practice to be ready for larger acts that you may need a press pass or whatnot to shoot. I am pretty much exclusively a fan of smaller, lesser known bands, so I have never had a problem getting into a show with my DSLR.
Some shots I've taken with 50mm and no flash:
1) Genghis Tron at the Starlite. This was the most difficult show to shoot. Their live performance is crazy with an intricate light show to match and I had a lot of trouble keeping up. The 100 or so sweaty metal fans bumping into me and squishing me into the stage didn't help

It was damn fun, though, and the shots I did manage to catch are some of my favourites. Don't be afraid of noise!
2. Nomeansno at The Pawn Shop. Easier to shoot as their lighting was pretty static for the most part. I was lucky enough to be on stage for most of the show, and able to move around a lot. Sweaty punk fans were my only obstacle for this shoot.