According to this interview with Harmonix, Rock Band is only going to be available as a bundle until February 2008:
"On the “items not sold separately until 2008″ rumors:
When Rock Band ships this November it will only be available in one flavor, bundled. Done. Confirmed. Although there are a few reasons for this the main seems to be based on retail. Firstly, the components in the bundle are not cheap and thus creating a bundle and three other separate packages (guitar, drums and vocals) would increase costs which could lead to less bundles available at ship. Lastly, although retail partners are confident the title will be successful they are appreciative of the fact that they won’t have to display four reasonably-sized packages in each of their stores. When is Harmonix looking to ship the separate components? Early 2008 is the general goal, so EB/GameStop’s leak was correct."
This and other information on Rock Band is available at this link.
"On the “items not sold separately until 2008″ rumors:
When Rock Band ships this November it will only be available in one flavor, bundled. Done. Confirmed. Although there are a few reasons for this the main seems to be based on retail. Firstly, the components in the bundle are not cheap and thus creating a bundle and three other separate packages (guitar, drums and vocals) would increase costs which could lead to less bundles available at ship. Lastly, although retail partners are confident the title will be successful they are appreciative of the fact that they won’t have to display four reasonably-sized packages in each of their stores. When is Harmonix looking to ship the separate components? Early 2008 is the general goal, so EB/GameStop’s leak was correct."
This and other information on Rock Band is available at this link.