I've asked Microsoft officials when they expect the final version of the Windows Phone Remote Desktop app to be available and whether they have plans to add support for Windows Phone 8. No word back so far.
Update: Windows Phone Remote Desktop will not support any versions of Windows Phone earlier than 8.1, Microsoft officials confirmed. The final version will be available by the end of this year, they added. And while Remote Desktop Gateway support is not currently part of the preview, Microsoft is not saying whether this feature will remain unsupported by the time the final ships. (But since the Remote Desktop app for Windows does support Remote Desktop Gateway, I'd say there's a strong likelihood the Windows Phone 8.1 version ultimately will, too.)
Windows Phone 8.1 adds a number of enterprise-focused features back to Microsoft's phone OS. Enterprise VPN, supporting IPsec and SSL VPN gateways; enterprise Wi-Fi; certificate management; S/MIME support for secure email; the ability to lock down documents in OneDrive via information-rights-management software and policies are part of the release. Some of these features require users to provision via a mobile-device-management product/service. But some others, such as EAP authentication for WiFi, also can be manually provisioned and configured.
Update: Several users are reporting that the Remote Desktop app for Windows Phone appears to be a universal app, in that it recognizes that they have already downloaded the Windows version, and that they simply need to "reinstall" the WP one.
(Thanks to Robert Brand for the tip on Windows Phone 8.1 Remote Desktop's availability.)