If you're interested in picking up Minecraft today you can get it $5 cheaper than the $20 Marketplace price from Best Buy. It's rare to see deals on XBLA codes after Amazon stopped selling them. It's even stranger for it to happen on launch day. This works both in store and on the website. The code should be delivered to your email, but it may take a little while to show up.
edit: The price is up to $19.99. Sorry folks. Deal is dead. People elsewhere have reported that they were still able to get it in store, but that's probably temporary as the stores update their prices.