The technology is by no means as advanced yet. The (active) GPS antenna
must receive an pretty unobstructed view of the sky, while remaining
mostly horizontally. Even most current GPS chipsets need lots of juice
(both the HF and the DSP part), and so does a cellular modem. Considering
batteries, this is a package 2-2 the size of a modern cellular phone --
nothing you would want to stick to your lapel.
ciao,
'gene