For the people whose Wearable is based on a PDA, there is little choice : most of the time they have to do with the PDA's manufacturer OS, AFAIK Linux on Psion series 5 is not mainstream. a commercial solution called WindStone primary targeted to palmpilots with a project to port it to ARM based computers it is made by OSK, so and see. For Mips based PDA type; some of them seem to be able to run NetBSD ( it is a member of the *BSD family of unices running on a lot of platforms ).
For those using a PC derivative, the choice seems to be between Microsoft products or Linux, Well, Microsoft products are not exactly the best choice and DEBIAN/GNU Linux is the winner ( I don't think that for general purposes one has to use a posix .4 compliant kernel ).
There is another contender QNX, but is more industrial oriented, so for hobby or research it is not the best choice.