When ESX Server 3i was announced in September 2007, Sun was one of the very few OEMs which didn't compare in the VMware supporting partners.

That was not too much surprising considering that Sun is coming with its own hypervisor, xVM Server, and may make the most of an exclusive hardware support to take a competitive advantage.

Now seems that Sun chanced its mind and it's ready to support ESX Server 3i on its servers.

It's possible that Sun is trying to maintain a good relationship with all virtualization vendors until it's competitive enough with xVM Server.
An interoperability agreement with Microsoft was the first step. This is the second one.