Here's the deal.
I, too, am a first time poster, because I am really upset about this.
I became a mac convert in 2006. In may of 2006 I purchased my first macbook pro, and in november of 2006 I bought my mac pro, which I'm still using today.
When the list of upgradable devices came out, I was not surprised to see that my macpro was off the list. I was actually okay with this - I had been thinking it might be time to upgrade that later this year. in the meantime, however, I have a mid-2009 macbook pro that I bought new in 2009. I was happy to see that one was on the list.
Like many of you, one of my main reasons for upgrading to Mountain Lion was because of airplay. I have an appletv2 that I have not decommissioned because of the airplay feature, and was looking forward to being able to utilize this on my macbook pro now.
I, like others, had no idea it woudln't work. I, like others, found this thread after googling and thinking I was doing something wrong.
My main reason for moving over to Macs is because of the fact that I can invest in a new machine once every 4 - 5 years and still be happy.
My macbook pro, with 8 gigs of ram, seems to do just fine running high end graphics programs. Heck - my mid2006 Mac Pro with 16 gigs of ram runs my graphics programs even better than my macbook pro does. (Adobe CS6).
It's not the $20. I feel mislead. And I am, for the first time, not a happy mac owner.