Having an iPhone version of GarageBand is almost irrelevant without iCloud functionality.
Apple did a great job tailoring the GarageBand UX to iPhone. If you are out and about and you have a musical idea that you want to quickly jot down GarageBand can work great for that. So I wouldn't say that iPhone support is irrelevant. As for the iCloud functionality, it's probably coming. The challenge here is transferring audio files back & forth between your devices and the iCloud servers. Audio files can be quite big, so iCloud support under 3G is unrealistic. iMovie doesn't support iCloud either and Photostream doesn't sync videos, only photos. I would guess for the exact same reason. The files are simply too big.
Apple could easily produce a second music making app and label it as GarageBand Pro or even Logic for iPad; I really do think that people could distinguish between these versions and grab the app best suited for them. It’s a necessary evolutionary step that will keep Apple relevant in the music production field for years to come.
GarageBand, whether for iOS or OS X, is a cut down version of Logic Pro. GarageBand for iOS is a big achievement in my books. Apple doesn't need to hurry up and implement advanced features, not just yet anyway. I think they are moving forward at a good pace. Surely, once iPad gets more powerful we'll see more advanced features.