"I had a terrible time synching [Mac OSX ] iCal to my Palm Treo. Other people report the process goes quite smoothly, especially using Missing Sync software from Mark/Space, but it didn't work for me. I ended up permanently scrambling my calendar, and needing to re-enter appointments by hand, which was no fun. (Ultimately, I bought an iPhone, but since this is a rather expensive workaround, it's one that I don't recommend to most people.)
I was at Fry's in Concord, California last night. Windows Mobile PDAs are not to be found. (There was one unidentified, non-operating HP iPaq.) A few pathetic-looking Palm PDAs were there as well. Sony's Mylo was there, but remember, it has no calendar, just a Web browser. Next week, at LinuxWorld, I'm meeting with PalmSource (now Access) as well as the Windows Mobile folks, Motorola, and the LiMo Foundation (working on mobile Linux). I really want to understand where offline mobile calendars head from here. The iPhone is forcing that conversation.