From my personal experience I think some numbers do get reused. When I first got my Google Voice number I kept getting a bunch of calls asking for some other person, so I guess Google just reused someone else's old number.
Numbers do get reused. I live in Paris and last year a friend freshly arrived from my hometown in Portugal was attributed the cellphone number (major French operator) from a german acquaintance of mine that had moved out of Paris a year before. She still gets text messages in German all the time.
Do they get reused eventually with the hope that the time since burning is long enough for the old usage of the number to have died off?
[edit] Sorry, didn't see PanMan had asked virtually the same question already.