Coding, testing, and supporting all these different scenarios, in multiple languages, presents some challenges for a product that is as flexible and capable as Lotus Notes. Given all of that, the existing Lotus Notes client code, including Domino Designer, will likely never be ported to a Linux client operating system.
That is too bad. My main reason for sticking to the Wintel platform is the Designer client. Anybody who knows the specification for communicating directly on a Notes socket?