The answer depends on how you got IHS.
Under some circumstances, your sales representative may be able to arrange for "standalone support" of IBM HTTP Server, which carries detailed terms and conditions.
When another vendor lists IBM HTTP Server as a "supported webserver", this is a statement of their own support position in terms of interoperability, and has no bearing on entitlement to support with IBM.
The generic proxy support is never supported as a frontend to WebSphere Application Server. The WebSphere Plugin is the only supported mechanism to connect IHS with WebSphere Application Server.
If your entitlement to IBM HTTP Server is via a WebSphere Application Server license, then IHS is licensed and supported only in support of WebSphere Application Server.
Some very limited reverse proxy (gateway) usage in such an instance of IHS can be supported, but not in the following cases:
Forward proxy is not supported when IHS is provided with WebSphere Application Server, even in cases where a customer writes an application that requires a forward proxy and deploys it to WebSphere Application Server.
An example of a supported usage would be to forward small ranges of the URI-space to another server during a period of migration, or to workaround virtual-host related configuration issues.
If you're using the free web download of IHS, IHS is may be used in any manner without support.
Finally, customers using Proxy functionality are encouraged to move to IHS 7.0 if possible. This is partially due to the higher quality proxy support in Apache 2.2.x and partially due to IHS 7.0 being more recently "rebased" then previous releases.
IHS is supported on any virtualization technology that runs a supported, unmodified, operating system. We assume any usage question or defect report is virtualization-agnostic until evidence shows otherwise, and won't ask customers to recreate an issue natively unless we suspect the problem is unique to the virtualization environment.
Technote #1242532 formally covers the virtualization policy for WebSphere Application Server (including IHS).We don't have any tips for the tuning the guest or hypervisor for IHS.