I hate to be the bearer of bad news.
It is true that the feature is not fully implemented in DP1, as Apple has not yet provided this functionality to all Macs with second-generation Intel Core CPUs.
However, Airplay Mirroring WILL require a second-generation Intel Core CPU upon 10.8's release.
Something similar happened with Airdrop in Lion. In Lion DP1, Apple noted that the feature was not yet fully implemented. Those missing the functionality were hopeful that "not fully implemented" meant that Apple would extend Airdrop to all Macs compatible with Lion in subsequent previews. It turns out that "not fully implemented" meant that Lion DP1 did not provide Airdrop to all Macs capable of simultaneous infrastructure and personal area ad-hoc networking, which is the requirement for Airdrop. Apple did bring Airdrop to THOSE Macs in subsequent previews, but Macs without the required wireless chip did not gain that ability.
Apple's implementation of Airplay Mirroring relies upon Intel Quick Sync, which includes hardware H. 264 encoding. Second-generation Intel Core CPUs have it onboard; Intel Core 2 Duos do not. In a future DP, Apple will bring Airplay Mirroring to those Macs with second-generation Intel Core CPUs that did not have it in DP1. Those with Core 2 Duos won't be getting the feature.
In my opinion, Apple has at times over-engineered a feature to exclude Macs that should in theory be perfectly capable of it. Honestly, I'm not thinking that this is one of those times, though (and while I'm a huge Apple fan, I'm certainly no Apple apologist). I feel really badly for someone that bought a $2800 MacBook Pro at the end of 2009 who, not even three years later, is finding him- or herself "left out." But I think that in the case of Airplay Mirroring, The Powers That Be asked themselves, "Do we pick greater compatibility or the best user experience?" No stranger to trade-offs (*cough* iPhone 4 antenna *cough*), Apple picked the best user experience. And as anyone who has used 10.8's Airplay Mirroring will tell you, it is WICKED fast.
If anyone is wondering how I know about Airplay Mirroring's requirement--this is what Apple told the journos and devs invited to see 10.8 the week prior to seeding. That I know of, among the journalists invited, only Jason Snell bothered to note this requirement in his write-up:
http://www.macworld.com/article/165407/2012/02/hands_on_with_apples_new_os_x_mountain_lion.html.
Hope this clears everything up.