I agree with Zack that simply accepting anyone for 90 days could, and most likely will attract more abuse than anything else.
Though, it could work out okay if the 90 days was clear and you required a valid CC or paypal account with signup, and possibly domain only hosting. You could also limit to quality sites only, which gives a better chance of future upgrade in 90 days.. provided you supply them with satisfactory service over that time of course.
Forcing an upgrade is not something i would like as a customer, and wouldn't do as a webhost however.
-- Chris