Gamespot has posted a review (8.5/10) of the first DLC for Borderlands. The Zombie Island of Dr. Ned is currently available for the PS3 and Xbox 360, with a PC installment "coming soon"
According to the review, the difficulty level is scaled to your progress in the main storyline and not your character's level.
Amusing recordings give you silly glimpses of other adventurers, and a cheerfully coldhearted company representative reminds you to never, under any circumstances, engage in oral contact with the undead. Dr. Ned himself has some great lines, and he preserves his general apathy throughout the revealing campaign to hilarious effect. This liberal dose of humor, combined with the new environments, helps create a healthy variety both within the add-on and when compared to the main game.
According to the review, the difficulty level is scaled to your progress in the main storyline and not your character's level.
To get loot and face enemies commensurate with your level, it's important to keep the following in mind: The Zombie Island of Dr. Ned scales everything according to your progress through the main story of Borderlands, not according to your character's level. So before you head to Jakob's Cove, take stock of your situation. If the enemies and the level of your current main story quest are about on par with your character's level, then you'll find a good challenge and good loot on the Zombie Island. If you've been spending time leveling but not advancing the story (that is, by doing lots of side quests or playing cooperatively with someone at a different point in the story than you), you may be overleveled for your current story progress. This could make Zombie Island too easy and make your loot haul unsatisfying.