Here's one more thing to consider. If Bibby does not opt out, and Artest does (both, fairly likely scenarios), we shall have a committed salary of around $49M in 2008. Martin shall be starting the first year of his new contract and we shall also be adding the salary of our 2007 and 2008 draft picks. After all this, we won't have the money to give Artest an extension he shall expect (even if he wants to stick around), unless the Maloofs agree to get into LT territory (if we sign some free agent to MLE this year, then it gets worse).
Admittedly, lot can change till that time. Bibby will be in the final year of his contract, and might become attractive to teams, allowing us to clear some space by then. However, granting a big extension to Artest, knowing that his good behavior might vanish before the ink on his contract is dry (allow me a little cynicism. Artest's past behavior has earned it), is, to put it mildly, tad risky. It might be a good idea to trade him, instead of watching him leave for nothing.
Miami and SA are two teams who need to win now, and can use him. Problem is getting something useful out of either of them (Heat is slightly better positioned than Spurs in this regard).