Harmonix has outlined the process by which musicians can get their songs onto the upcoming Rock Band Network.
Speaking to GameSpot, Harmonix's Greg LoPiccolo reiterated that the basic requirements are an Xbox 360, Xbox Live Gold membership, XNA Creators Club membership ($99/year), Reaper and Magma programs and a bunch of spare time.
Once you have written a song you want on the Rock Band Network Store in real life, you can download the Rock Band specs from the Network website and read up on how to do it, but in short, you need to separate out the multi-track recordings of your songs, mix them into stems that match the RB specs, use a MIDI sequencer (Reaper or equivalent) to chart the note information, load all that stuff into Magma for error-checking and transfer to Xbox 360, and then play-test it yourself.
Once you've done all that, you upload the result to creators.rockband.com and other people can download and evaluate it and provide informal feedback. Once you're happy, you can submit it for formal peer review, and providing it doesn't infringe copyright, swear absurdly or get the Xbox 360 disc tray to jump in and out of the machine inappropriately, it will be tossed up on the Rock Band Network Store so people can buy it.
LoPiccolo points out that there are already people on the Rock Band forum who understand the process offering their services to less technically proficient musicians, but warns that the person who submits the songs has the rights to it and gets the royalties, and any other agreements should be made between individuals. As it's not possible to automate very much of the process, budding sellers will need to know a bit about making music and getting computers to understand it, to say the least.
The Harmonix man also notes that the service, which goes into beta in late September, will be US-only on the technical side to begin with, with songs made available in the US, Canada and Europe (except Germany). The latter restriction is down to the availability of XNA, which should expand in future.
Oh, and it sort of goes without saying, but this is not the same thing as Guitar Hero's Studio mode, so your dreams of making millions from an evening in with a plastic guitar, rubber drums and Pinot and Grigio will never be fulfilled. Sorry.
Harmonix has previously said that the service will be exclusive to Xbox 360 for now, but that super-awesome songs may be transitioned to PS3 and Wii.