Based on our experience in January '07, if the "Park Rangers" see that you are a climber (can see your gear on your back or in your vehicle) when you enter the park, the rangers will tell you that you need a "guide". We'd been warned this would happen...
But it just so happens that there will also probably be a park-approved "guide" waiting there at the entrance who will take you into the park for something like $40(?). He'll be a "naturalist guide" and he'll point out the wild horses, flowers and trees as he drives you to the hut (almost). We climbed Cotopaxi without a mountain guide, AND I can tell you all about the local flora and fauna of Cotopaxi National Park.
If you need or want a real guide, make arrangements with Moggely before you go. If you don't need a guide, welcome to South America.