Wow, that really concerns me now. There are only a few obvious reasons I can think of to do that:
1) PalmSource is dying
[Don't think so, and sure hope not!]
2) PalmOS is going whole-hog into phones, so a devcon is a bad idea right now for traditional pda developers [possible]
3) Timelines are slipping on PalmOS for Linux and they don't want to reveal the problems publicly [also possible]
4) They are so focused on phone linux right now that they just don't want to take the time out to deal with a devcon [that sounds crazy to me]
5) They are way ahead of schedule and PalmOS is going to be rolled out a month after the DevCon scheduled date, so they don't want to spoil the thunderous surprise announcement. [I can dream, can't I?

]
Any other ideas?