Buying domains that include a brand name is not uncommon. Protecting brand identity is becoming more and more important on the Internet, and those "in the know" spend time and resources researching and buying as many brand-inclusive domains as possible. Google and Yahoo! purchase domains on a regular basis (I am sure MSN does too), and some people that keep an eye on these sorts of things can easily start rumors. Some rumors end up turning into reality, which is what makes them fun to discuss.
A recent thread at Digital Point Forums is a good example of a hypothetical discussion based on the Digg of a report of a domain name purchased recently by Google: Googlemastercard.com. The thread reveals some theories that may not be all that far from the truth, including an eventual Google Debit/Credit card. One of the cooler ideas I saw was that G would simply credit your account with funds earned from AdSense, making for easier access to that hard-earned publisher fee.
See the thread at Digital Point Forums.
For more great rumor-fodder, you can see Gary Price's recent posts at Resource Shelf, including three lists of Google buys and a Yahoo! list too (Google Domains, More G domains, more G domains, and Yahoo! Domains). I actually just started a couple threads at Search Engine Watch Forums for the further discussion of these lists posted by Gary. (Google thread - Yahoo! thread)