I think that out of possible MMOG payment models, product (or presence) placement shows most promise.
The energy which sustains the world has to come from somewhere, and it lessens needful dependency if it is one with it. (It's nicer if there are visible links.)
Eventually, if your world matures and becomes self-sustaining, you can set it loose.
Transformation is the key tool for successful product/presence placement.
Initially, there are two types of product placement: product placement (where you develop a product in the real world and place it in the game world) and inverse product placement (where you develop a product in the game world and place it in the real world).
Eventually, these two types would merge together.
How to do it right?
Take the product (or service, company, brand name etc.) and strip it down until it's completely bare.
What does the product stand for? What is its significance in the real world? What are the motivations, ideas, inspirations behind it?
What statement does it give?
What you now have is the product's energy signature; a thin thread pulsing with the lifeblood of the world.
Choose carefully where you shall place it and how it shall manifest.
If there is synergy, it can make it's statement without grabbing unnecessary attention.
Weave the threads attentively, with clarity and Love.
Will you use few coarse threads (select companies) or more fine threads (MMORPG meets Google Adsense)?
Take a step back and observe.
If you don't Love the world, you'll see no more than the sum of parts - you have failed.
If you find that your world is breathing, you have succeeded.