Microsoft has published a 90-day trial of Windows 8 Enterprise RTM at MSDN, anyone with a Windows Live account can download it at Microsoft's website.
This evaluation provides a 90-day trial of Windows 8 Enterprise edition. Both 32-bit and 64-bit versions are available as ISO images in the following languages: Chinese (Simplified), Chinese (Traditional), English, English (UK), French, German, Italian, Japanese, Korean, Portuguese (Brazil), and Spanish. In order to use this evaluation, you must register and the product must be activated online with Microsoft-hosted activation and validation services. Microsoft does not provide technical support for this software.
Upon installation, you will have 10 days to activate the trial version online. Once activated, you have 90 days to test the software. Afterwards you need to perform a clean install because upgrading isn't possible.