If you're familiar with Unplugged, you should be able to jump right into Rock Band 3's Expert modes on DS. The gameplay is identical: all the instruments and the vocals are charted at once, and you freely switch between them as you attempt to successfully play note phrases to keep each part "alive." It's Harmonix's classic Frequency game, but with rock music. Rock Band 3 for DS doesn't take advantage of the touch screen or the built-in mic -- nor does it require you to pay attention to both screens at once. The top screen displays your characters performing, while the bottom screen is the familiar note stream.
Nearly all of the features of Unplugged make a reappearance, but some are given new names. For example, "Band Survival Mode" is now called "Pro Mode," adopting the moniker given to the console game's biggest new feature (but you won't be plugging your MIDI instruments into the DS). Additionally, The Tour and Career modes have been enhanced in the some of the same ways as the console version: there are now "unique gameplay challenges" (objectives) that you'll need to achieve to progress.
Undoubtedly, the DS game's biggest improvement over the PSP Rock Band game is the addition of both co-op and competitive multiplayer. Using the DS local (ad-hoc) wireless connection, two to four players can sync up and play music with each other. Co-op mode is rather straightforward, allowing everyone to choose an instrument and play it for the entirety of the song. Unlike the standard game mode, however, you won't be able to switch between instruments and there are no bonuses for completing phrases. Competitive multiplayer is a score-based affair, not unlike the competitive offerings of the console games.
Rock Band Unplugged (PSP)
While Rock Band 3 for DS has all the features we'd want in a mobile Rock Band game, it's definitely lacking in content. According to Harmonix, only 25 songs can be squeezed into the rather restrictive cartridge format. In addition, there's obviously no way to support DLC for the game. Compared to Unplugged's 41 tracks (and 57 additional downloadable songs), Rock Band 3's DS library definitely seems rather insufficient.
Still, when it's released on October 26, the game will be the only DS version of Rock Band out there, and with no more Unplugged DLC in sight (ever), handheld fans, myself included, will have no choice but to pick it up for more Rock Band on the run.