I think making Now API-interactable would just beg for spammy popups. Mobile is already a maelstrom of apps trying to promote themselves, or getting paid to promote others.
A Google Now API would be awesome.
Requires addressing in angles regarding security, UX, and performance at the very least.
[1]http://support.google.com/accounts/bin/answer.py?hl=en&a...
[2]http://www.androidguys.com/2012/02/28/airpush-ad-network-con...