Google said that in coming weeks, "eligible iPhone and Android app developers in AdMob network will be able to show AdSense ads when an AdMob ad isn't available." This'll also enable advertisers to extend the reach of their mobile campaigns to thousands mobile apps in AdMob's network, such as Rovio's 'Angry Birds'.
It's easier for AdMob publishers to access this new inventory. Publishers using a recent version of SDK willn't have to update their code. Reporting will integrate directly into AdMob account, and you'll continue to receive single check from AdMob each month.
Publishers should note they'll not be able to filter "types of AdSense ads" (e.g. category, URL) that run in their app. As a result, publishers who've ad filters selected for their AdMob inventory need to opt in to receive Google ads. Publishers can check their "App Settings" tab in Sites & Apps section of their publisher account to see if they're eligible for Google ads and opt in.
Eligible iPhone and Android app publishers not utilizing ad filters will be opted into receiving Google ads.
More Info: Help Center