Last season with the Brewers Mike Cameron wore number 25, he also wore that number with the Padres. That number currently belongs to Mike Lowell and since the trade has not become official it is unlikely they will give up Lowell's number.
When Cameron was on the Mets, Mariners, and Reds he wore number 44. Jason Bay wore number 44 for the Red Sox. If the team allows Cameron to take number 44 it will show that the Red Sox have no plan to bring back Jason Bay, but it also keeps them from driving up the price for other teams.
Cameron wore number 24 when he played with Chicago and that number is available, Saito wore it last season. It will be interesting to see which number Cameron picks and he may end up changing numbers once Bay signs elsewhere or the Lowell deal becomes official.
UPDATE: Camerson picks number 23.