Tiny and passively-powered radios would make for some interesting applications. One major issue is that you cannot shrink an antenna down infinitely; its size is dependent upon the wavelength of EM radiation that it is trying to detect. Researchers at Stanford and Berkeley have announced "ant-sized" radio-on-a-chip devices, fabricated at 65nm, which are powered by the signal that they gather.
The catch is that, because their antenna is on the order of a few millimeters, it is tuned for ~60 GHz. There are reasons why you do not see too many devices operate at this frequency. First, processing that signal with transistors is basically a non-starter, so they apparently designed a standard integrated circuit for the task.
The other problem is that 60 GHz is an Extremely High Frequency (EHF) and, with its high frequency, is very difficult to transmit over long ranges. The 57-64 GHz region, in particular, is a range which oxygen resonates at. While it is possible to brute-force a powerful signal through a sensitive antenna, that defeats the above purpose. Of course, the researchers have been honest about this. Right in their IEEE abstract, they claim a current, measured range of 50cm. In their Stanford press release, they state that this is designed to be part of a network with units every meter (or so). Current bandwidth is a little over 12 megabit.
Simply put, this will not become your new WiFi hotspot. However, for small and connected devices that are in close proximity, this could provide an interesting communication method for when size, cost, and power efficiency trump speed and range.