There's no universal "generic" Android that will just automatically work on all phones, due to differing hardware. Some customization is necessary. That said, ROM hackers have been working hard for a long time now taking ROMs for other Galaxy S model phones and making the necessary tweaks for them to work (or work well-enough with minimal issues) on the Captivate. But they have to use these other ROMs as a baseline, and are at the mercy of what gets released for these other models.
Certain Captivate-specific features are often victims of this however and are lost, such as the Audience chip (voice processor, reduces background noise while speaking).
