I recently acquired a Redfly Mobile Companion from Celio. This device provides much (but not all) of the functionality previously promised by Palm for its never-released FOLIO. The Redfly, the size of a very small laptop, provides a larger screen and keyboard for using a mobile phone. It is ideal for taking advantage of unlimited on-device data plans. Since the Redfly does not contain any memory and all processing is done right on the phone, if the Redfly is stolen or lost, the data remains secure (so long as the phone is not lost). Battery life seems very decent (and can even be used to recharge or continue powering a depleted cellphone).
My big disappointment is that the device is not compatible with cellphone sold by Canadian carriers, particularly Rogers. Hopefully this is a short term issue and Celio will update the device in the near future.
Postscript: according to Celio, the compatibility issue is limited to phones running Windows Mobile for Smartphones (as opposed to the PocketPC version of WM). The Redfly software needs low-level access to the firmware, which in WM for Smartphones , can be protected unless the application software vendor has obtained a certificate from the carrier. According to Celio, Rogers is not being cooperative in helping Celio customize their software for us on smartphones sold by Rogers.