From Jayski's Silly Season Site - NASCAR Frequently Asked Questions:

"NASCAR owns and assigns car numbers to an owner. NASCAR reserves the right to revoke or transfer car numbers to another owner/team at any time. Car numbers are non-assignable and non-transferable, except by NASCAR. NASCAR works with teams and sponsors when a specific number is requested. If duplication of numbers occur, which usually happens with the part-time teams, the fastest qualifier usually gets to use the number unless one team is considered full-time. In 2002, the #27 was used by Scott Wimmer for Bill Davis Racing and by Kirk Shelmerdine, both part time, however, Wimmer was listed in the #27 and Shelmerdine in the #172 and when they attempted the same races, Wimmer got to use the #27 and Shelmerdine ran the #72, so NASCAR uses its judgement in deciding who runs the number."

huh. never thought about it!