The PalmPilot family and its clone is a hit among PDAs, but less known is the fact that it is more than a PDA it is one of the first off the shelves wearable. You can connect it to your cell phone, When on the move you may need an input device more convenient than Graffiti: a keyboard for example, one device exists that allow you to plug a PS/2 keyboard in your palm: Happy Hacking cradle Specially for the Palm.
PFU Happy Hacking sells a PS/2 adapter for the Palm, thus one should be able to hook any PS/2 keyboard on this device. It is battery ( 2 AA ) powered.
It seems that someone has managed to get a twiddler keyboard working with the PalmPilot. If you want to do land navigation it exists an electronic compass you can plug into your palm it is called Palm Navigator and it is manufactured by Precision Navigation They even sell a weather station.
If you have more money to spend or want more precision you can purchase a GPS receiver called Earthmate; it is manufactured by Precision Navigation Some of you may want a software only solution: this can be done. If you have some highly specialized needs like commuting without having the subway map and want a readily made solution you can use a program like Route Expert, I use it on a daily basis, it works great with Paris's subway and I will test it with Berlin's subway at the end of the year ( of course it exists other programs but this one is the one I am using ). I guess everyone can find the North using the sun and a wirtswatch, a PalmPilot can do it with more precision using readymade programs such as T.J's Sun-Compass ( once again there exists other programs but this one is the one I am using ).
The most important with the PalmPilot for the Linux community is the fact that there exists two ports of Linux for it ( DragonBall is a member of the 68000 family after all ). One is GPLed
You can find more information at : uClinux
the second one is commercial, it is based in Korea. WindStone it is based on uClinux and it is coming with its own GUI and widgets.
Thus with the next generation of Palms, the CPU speed will be twice today's speed, and with Linux on board we will be able to run a lot of software that are hardware independent and have been written for today's x86 Linux based wearables.
Samsung displayed a PDA named the Yopi runing Linux. It is an ARMLinux based PDA, 32 MB RAM, featuring a voice recorder, mp3 player,IrDa port, compact flash slot, a 4" color TFT and HandWritting Recognition, plus Serial and USB interface. Word has spread that it should be sold this summer but unti then wait and see.
It exists a Mips based PDA info Center This site focuses both on NetBSD and LinuxCE on MIPS based PDAs.
You can find some information on the LinuxCE port; and you may wish to read the LinuxCE FAQ With theses PDAs one should be able to consider building a nice PDA based Wearable.