As stated in another thread on alldeaf before, ZVRS would love to, but cannot release an EVO version at this time due to exclusivity reasons between Mirial and MainstreamVRS. I have no doubts that the exclusivity was not cheap for MainstreamVRS, but anything I have to say toward that end would be hearsay. No exclusivity arrangement is forever, though.
The MainstreamVRS app and the Z4 are essentially the same Mirial android softphone, with different branding, and different artificial limits imposed by Mirial blocking operation on other phones.
If you try to run the Z4 Mobile app on an EVO, you will get the message:
"Your phone model (PC36100) is not supported. This software only supports
the Samsung Galaxy S, Samsung Epic, HTC Desire, Nexus One phones"
This means that Mirial semi-officially supports it on the HTC Desire and Nexus One, though there is no front-facing camera on those phones.
It also works on a number of other android phones, though the front-facing camera is not going to work on those either until Mirial adds it to their list of supported chipsets. This will happen over time.
The problem is that Android 2.2 Froyo does not have an official front-facing camera API. There have been rumors that they will in the next version of Android. Until then, every device manufacturer must supply their own API, and any application developers must attempt to get their applications to work with these manufacturer specific APIs.
It's more difficult than you might think.