I just tried creating a trial account for the Enterprise edition using the http://www.salesforce.com/products/editions-pricing/feature-comparison/
I followed the steps mentioned below to see the trial account created for Professional edition
a) Clicked Enterprise Edition text button(in Editions and Pricing section).
b) Clicked Free Trial and followed the NEXT STEPS section entering all the details and clicked Submit button
So, please guide me to create an trial account on Enterprise Edition.