This was the text of a reply from UAD Support:
Currently we are still advising that Cubase users stay on 6.5 or use the following work around:
Q: Does UAD version 7 work with Steinberg Cubase 7 and Nuendo 6?
A: We are working directly with Steinberg to resolve a Cubase 7 and Nuendo 6 issue with UAD Powered Plug-Ins and we currently do not recommend using these hosts with any version of UAD software. UAD Powered PlugIns version 7 is fully qualified with Cubase 6.5 and Nuendo 5.5.
The issue: Sample rate and/or buffer size errors can occur when using UAD plug-ins and Cubase 7/Nuendo 6 is set to anything other than a sample rate of 44.1 kHz and an I/O buffer setting of 1024. After the error occurs, the UAD plug-in does not process audio and “disabled” does not appear in the UAD Toolbar, so there is no way to determine if the plug-in is functioning (other than listening to it).
Note: If Cubase 7/Nuendo 6 are constrained to use only the 44.1 kHz sample rate AND the 1024 I/O buffer size, UAD Powered Plug-Ins may work, but again these hosts are unqualified and are not supported.