I don't carry a sharpening tool around daily. I do pack a Spderco double-stuff combination stone with me whenever I am away from home for several days, or am going backpacking for any more than a day. The stone is ceramic, with two grits on either side of a 1 X 5 X 0.25 inch long slab. Grey, a medium grit stone on on side, and a white fine stone on the other, all you will ever need for some serious edge work on carbon or stainless steels.
It comes in a nice smooth leather slip sheath which makes carry easy. I coated the back (non-engraved with the Spyderco logo) with green chromium oxide stick, and use it as a small strop with the stone inserted in the sheath to provide a good hard surface for the leather. This small stone and strop combo works very well for me, and I prefer razor sharp edges at all times.
However, with the really good steels available in both custom and production knives (ATS34/154CM, BG-42, and 440V and 420V, for example), it is hard to see why one would have any real need to carry any sort of sharpening implement or device while going about routine daily tasks.
My double stone rides in my backpack when it is not on my desk. Not my pocket.
Paracelsus