As far as I know NICE have never certified 8.8 or 8.9 products on virtual server and when we requested it on behalf of a customer we were quoted $10k per piece of software to carry it out.
When we received that quote we tried the CLS and SC software on virtual server in our lab (can't remember what version) and had some success but we did see issues with logger communications and throughput on SC when we loaded the system heavily (CLS and SC were running on the same physical server). In the end the customer moved away from the idea of virtual servers and put in two dedicated HP's.
Having said that, we've currently got a training system consisting of four virtual servers running 8.9 CLS, SC, Avaya T-Server simulator and CA-Unicenter running on a single HP server under Virtual Server 2005 (all virtual servers are running Windows 2003) with no apparent problems. This is set up generate around 100 call events per hour and it doesn't seem too stressed but I don't think I'd want to try it on a customer site!