The Mojang blog follows up on the recent dustup over commercial Minecraft server operation with a clarification of their position of what server operators are allowed to do. Here are the does and don'ts:

You are allowed to accept donations
You are allowed to accept donations from players. You can thank them publicly or in-game, but can’t give preferential treatment for donating. You are not allowed to restrict gameplay features in an attempt to make money.

You are allowed to provide in-game advertising or sponsorship opportunities
You are allowed to put adverts in your Minecraft worlds to help with costs. If used within reason, adverts and sponsorship can be appropriate ways to fund a server.

You are allowed to sell in-game items so long as they don’t affect gameplay
We don’t mind you selling items in game, but they must be purely cosmetic. Pets, hats, and particle effects are OK, but swords, invincibility potions, and man-eating pigs are not. We want all players to be presented with the same gameplay features, whether they decide to pay or not.

There is one exception to this rule – capes! We have a lot of fun making cool capes for extra-special members of our community and Minecon attendees. We’d like to keep them as exclusive as possible. So, yeah, no capes please, even if you’re giving them away for free.

You cannot charge real-world cash for in-game currency
We don’t mind in-game currencies which are earned through playing, but you are not allowed to sell them for real-world cash. Remember – if the stuff you sell affects gameplay, we’re not cool with it.

Don’t pretend to be us. Provide your customers with loads of info
If you do decide to monetise your server, you must clearly state that the purchase is not associated with Mojang, declare who the money is going to, and provide a purchase history and contact details. You should also check the legality of selling digital items in your specific region.