Rock Band is now available for purchase for $9.99, which seems to be EA's starting price point for most new release iPhone games.  It appears to play similar (from the screenshots at least) to the recently released Tap Tap Revenge 3 which only cost $0.99. Here is the feature list for EA's Rock Band:

• Go solo, or push it to the limit with LIVE BLUETOOTH MULTIPLAYER
• Sync up via Facebook on your own time.
• Rock out on guitar, bass, drums or vocals to MP3-quality songs
• Play at three levels: easy, medium or difficult
• Play solo, or start a band with friends from around the world
• Jump into Quickplay for casual sessions or take it on the road in Tour Mode
• Play along with 20 legendary songs from great bands like Smashing Pumpkins, Lynyrd Skynyrd, Foo Fighters, Blink-182, The Beastie Boys, and more!
• Start out playing in grungy dives and work all the way up to stadium-rock stardom
• Collect rewards and achievements that provide replay value and depth to even the most experienced Rock Band veterans
• Check out the in-game Music Store to purchase additional tracks

EA also offers Rock Band Unplugged currently for PSP at $39.99, but the gameplay experience is handled differently than the iPhone version.  I'm a huge fan of Rock Band, but I'm not sure what platform I would prefer to play Rock Band on the go.  I've played the demo on the PSP and enjoyed it, but there is no demo available at this time for iPhone (and I doubt there will ever be one).  I'll wait and see as some of the reviews start coming in before I make my decision.