I dunno when they changed it, but I got an email just now suggesting a bunch of stuff I don't understand, but ALSO suggesting that I could put more mobile ads on my sites (where were they when I was peak season) I was gonna email them about this, but then I thought I'd go double check the policy, and sure enough, it's changed.
If you're serving mobile ads for WAP devices (non-smartphones) you can still only serve one ad, but for "high end" devices (i.e. smartphones, i.e. 320x50 ads) then AdSense for Content criteria apply - so up to three ads.
Publishers should use standard AFC ad units for websites formatted for High-End devices. In this case, they should adhere to the AFC ad limit guidelines which allows up to 3 ad units per page.
This is welcome news, but I dunno why they don't announce stuff like this.