My Response: Note this question and answer is from 2011
It's common for various companies to offer one way auto rentals to relocate vehicles in
the spring from the south to the northwest. These one way specials generally take place from the end of March and can continue until the end of May or June, depending on when the rental company gets all the cars moved that they want.
Generally we see these yearly -- special rates from the south for pickup locations in Florida and sometimes Arizona.
I've done some research and have found the 2011 one way car rental companies and deals for spring relocation that are being offered now : (note: this answer was specific to the deals available in 2011, but the general time frame is still common... renters should check with their provider of choice to see current deals at that time of year)
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One way rental cars available for pickup up between April 25 - May 31, 2011 from Daytona Beach, Ft. Lauderdale, Ft. Myers, Melbourne, Miami, Orlando, St. Petersburg, Sanford, Sarasota, Tampa and West Palm Beach,
Vehicles must be returned at a participating Alamo locations outside of the state of Florida with the excpections of the following: no returns permitted at Alamo locations in Alaska, Arizona, California, Hawaii, Nevada, Oregon, Puerto Rico and Washington.
One way car rental deals are available to book for pick up between March 27 - May 31, 2011 at select Florida locations with the ability to return to select Avis locations in Arkansas, Georgia, Louisiana, Mississippi, North Carolina, South Carolina and Tennessee.
Avis is also offering one way rental cars for pickup between April 10 - June 30, 2011 at participating Avis Arizona locations to return the vehicle to a participating Avis location in Arizona, California or Nevada.
one way auto rentals must be picked up between between March 27 and May 31, 2011 at a participating Arizona locations and dropped-off at a different, participating locations in California, Nevada and Arizona.
One way car rental deals are currently available for rentals picked up between April 25 - May 31, 2011 from Daytona Beach, Ft. Lauderdale, Ft. Myers, Melbourne, Miami, Orlando, St. Petersburg, Sanford, Sarasota, Tampa and West Palm Beach,
Vehicles must be returned at a participating National locations OUTSIDE of the state of Florida with the following exceptions: no returns permitted at National locations in Alaska, Arizona, California, Hawaii, Nevada, Oregon, Puerto Rico and Washington.
While these are not the only one way car rental companies (different companies offer different specials at different times) -- these are the currently available one way car rental deals available for this time of year for pickup in the South and for dropoff in the Northwest.