The physical dimensions of the Visor(4.8" x 3.0" x 0.7") are very close to the Palm III series (4.7" x 3.2" x 0.7"). The IR port on the Visor is on the left side, so beaming while in the case may be difficult for current case designs. I emailed several case venders to find out if any of their current cases would fit the Visor, and also to find if they were working on any Visor-specific cases.
RhinoSkin- The entire line of RhinoPaks will accommodate the new Visor. There are plans to have a Visor specific case available (with minor changes) when Visor launches.
E& B Cases - No information was provided regarding compatibility with current models, but E & B is planning on a case for the Visor.
I2i UK- None of their current cases will work with the Visor, and there are currently no plans on making any Visor-specific cases.
PenComputing Designs -- Pen Computing will not be making cases anymore; therefore, there will be no Visor-specific cases from them.
Targus-- Targus has a Handheld PDA compatibilitychart that shows that all of their current cases are Visor-compatible. No word on if Targus is making any Visor-specific cases; most likely they are not, since most of their cases try to be universal in size.
The Handspring Visor appears to befairly compatible with current Palm cases and software. Not everything is compatible, but most case companies that I spoke with are working on Visor-specific products. I was not able to get in contact with all case manufacturers, so if your favorite case company is not listed, that may be why.