With regard to the comment about differing rates by different cab companies, technically (actually, from a legal perspective) that should not happen as the TAXI rates are controlled by the county taxi commission and are standardized. As of 11/21/2009, the Arlington county rates are:
$2.75 Initial fee (includes 1st 0.2 miles)
$1.75 Airport surcharge (if you use the airport taxi line)
$0.40 for each additional 0.2 miles (or $2.00 mile)
$1.00 for each passenger over 1
$0.50 Each suitcase in excess of two if handled by the driver ("footlockers" are $2)
$0.40 for each 64 seconds of waiting time, calculated fractionally (I think this includes being stopped at a red light or stopped in traffic - the meters auto flip to waiting when the cab stops moving)
IIRC Alexandria cabs have a different fare schedule and can pickup at DCA and take a passenger anywhere in VA/DC/MD. I do not think that Fairfax cabs can do this and know that MD and DC cabs will only take passengers to their own counties for MD cabs and DC for DC Cabs.
If you know what cabs go where the system actually does make sense but for the occasional visitor it gets confusing which is why its a good thing they have the dispatchers.