According to Percy Allen of the Seattle Times the Sonics asked for, and were granted permission to speak with Dallas Mavericks assistant coach Sam Vincent last night.
This same article declares Rick Carlisle, Rick Addleman, and Dwayne Casey as the frontrunners for the job. IMO Paul Silas should probably also be added to that list.
Sam Vincent coached one season with the Mavericks but his primary experience comes as a CBA coach. While I believe it is always hard to judge an assistant's level of contributions to a team he does seem to be ligtly qualified and would likely be an inexpensive hire.