For a sensor network project I have been using a modified Linksys wrt54g (wrt54gs) wireless box. (For a podcast on modding the wrt54g wireless router to run your own software based on linux, you can also see the related podcast on iTunes.) The driver on these devices returns a measurement of signal strength called rssi (Recieved Signal Strength Indicator). To access it you run a shell on the device and enter "wl rssi". In several places, this value is documented as being in dB (or dBm), which is a logarithmic scale for measuring power. The dBm unit is used for the power ratio in decibels (dB) of the measured power referenced to one milliwatt. I don't believe it is computed correctly of some software packages, such as mine. (For those running openwrt, ddwrt or similar, download the "wl" application and try (wl rss and wl noise). I several places RSSI is also formulated as the ratio of power levels. In practice, RSSI seems to actually be an ad hoc software-dependent signal strength measure, at least in order software.
The larger (closer to zero) the RSSI value is, the better the signal is. For example, --50 dBm is better signal strength than -80 dBm. In terms of absolute values, bigger is "badder."
RSSI is a classic way (albeit ad hoc?) of measuring the signal strength for a wireless devices (such as an 802.11b access point). As far as I know, it is not actually reported in dB, but some more-or-less liner scale. In fact the wrt54g "wl" command itself claims that RSSI and NOISE are reported in dB, but observed data contradict this. Note also that...
Typical acceptable values for rssi on the wrt54g and wrt54gs are between -70 and -90. Noise should be 5 or more units less than rssi but even when noise and rssi are about the same the device may work well.
Further, note the following quotes on RSSI which does not explicitly say whether log or linear scales are used, but which imply a linear scale:
"…a mechanism by which RF energy is to be measured by the circuitry on a wireless NIC. This numeric value is an integer with an allowable range of 0-255 (a 1-byte value) called the Receive Signal Strength Indicator (RSSI)" [IEEE 802.11 standard ]
"…Received Signal Strength Indicator (or Indication): A signal or circuit that indicates the strength of the incoming (received) signal in a receiver. (The signal strength indicator on a cell phone display is a common example). RSSI is often done in the IF stage before the IF amplifier. In zero-IF systems, it is done in the baseband signal chain, before the baseband amplifier.
RSSI output is often a DC analog level. It can also be sampled by an internal ADC and the resulting codes available directly or via peripheral or internal processor bus. " [Dallas Semiconductor ]