Both GOOG and MMI have clauses to opt out of the buyout. Each compnay would have to pay the other a large sum to excercise this right. Then you have potential regulatory issues; the deal could get blocked. I don't think it's likely, but you never know.
The other aspect is time value. If you have $38 tied up in MMI, that max payout you will receive is $40 and nothing more. Using those figures that's roughly a 5.25% gain. But this deal will not get approval for at least 6-8 months and possibly even longer; 12-16 months. During that time your money has the potential to return much more than 5.25%.
GOOG said they expected the deal to close in early 2012. I would think the end of February to the middle of March is probably the likely outcome. I could easily see it going further out. I expect mid January at the earliest. China will take their sweet time. TXN buyout of NSM was announced 4/4/11 and was okay'd by the DOJ on May 23. China will take their time on MMI probably too.