if msc is to be traded on centralized exchanges, what is the best way to make deposit/withdraw work ? Is it necessary to check if the input does not contain some withholding amounts from saving addresses ?
Probably, but since savings addresses haven't been implemented yet, that's not really a concern at the moment.
I have read that Mastercoin will take in price updates in order to allow decentralized betting on other commodities etc.
Will we also be able to use Mastercoin to gamble on discreet outcomes, eg Intrade style betting on sports / politics etc ? How will Mastercoin get the feed for that?
Any user can create a data feed about anything, including sports/politics, which people can then bet on (please follow the laws in your local jurisdiction!)
Strangely enough, we just changed that link to point to a wiki page instead with a list of brokers (
http://wiki.mastercoin.org/index.php/Buying_and_selling_Mastercoin), now that there are two (btc-market.net is the other one). Note that the change to the webpage seems to be propogating slowly across various caches.