I downloaded the Beta and installed Ubuntu One. There is a "No Entry" sign on the UO logo, try to run but gives the above error message. This is an intel mac book pro with a core duo and MAC OS V10.6.8
unfortunately Ubuntu One for Mac OS does not currently run on 32-bit processors like the Core Duo. This is not a fundamental limitation, but just a lack of resources. See this launchpad bug if you would like to vote to give this issue higher priority.