No, the Karma does not work with iTunes. I usually keep the Karma ethernet attached and use RMMLite or JEmplode to send music over to it. Firmware upgrades do require a Windows machine however, but that does include working from VirtualPC.

Honestly if you like using Linux, a Powerbook should be a good fit. You get some awesome apps out of the box for doing mundane things like photo managment, plus can bring over most Linux apps using Gentoo for OS X or FINK.

Visio might be the problamatic application. I know there are several similar apps on the Powerbook (Including Omnigraffle that may come with your Powerbook, it did mine), but I don't know if any have Visio compatibility.

*Edit* Just read the Omni page closer, it looks like Omnigraffle Pro can read and write to Visio 2002 files. Guess there is some good coming out of the move to XML by Microsoft in their Office apps.