HTC has confirmed via its official Twitter account that the new version of HTC Sense that'll be shipping on the EVO 3D, Sensation and Flyer won't be ported back to any earlier devices. Replying to a question from a Desire HD owner asking whether his phone would get an update to Sense 3.0, HTC said, "due to Sense's hardware requirements, only our newest devices (Flyer, EVO 3D, Sensation) will be able to support it."
This will come as a disappointment to some, including posters on our own forums who were hoping to see Sense 3.0 on recent phones like the Thunderbolt, but it's not really much of a surprise. Sense 3.0 is clearly more hardware-intensive than earlier versions, and the devices which will run it out of the box are a step beyond the current crop of HTC phones in terms of performance. Even the slowest Sense 3.0 device, the Flyer tablet, runs at clock speeds 50% higher than those of the Thunderbolt.
Let's also remember that HTC is by no means abandoning its Sense 2.0 (or even 1.0) line-up. Just because the Thunderbolt and Incredible S aren't invited to the Sense 3.0 party, doesn't mean they won't be getting Gingerbread later in the year. [@HTC via Unwired View]
Update - HTC tweeted a little clarification on this. Still pretty vague but looks like they are aiming to bring some of the aspects of Sense 3.0 to previous devices as well.