That's a complicated tale. I'm not really sure about this, but I think the actual coders for Notes never were Lotus employees: they are a separate company. I daresay IBM management would probably like a Notes client on Linux, but the Notes coders keep saying "no way". They cite the fact that converting Domino (the Notes server) to Linux was far harder than they expected.
When I was supporting Domino (on Linux) I and a few others kept asking in the Lotus forums when a Linux client would be coming out. It was getting uncomfortable for Lotus to keep saying "probably never". But they wouldn't budge.
Wade.