With unrelenting pressure on game prices in the App Store, developers are looking for additional ways to make money. It comes down to two options: iPhone 3.0's in-app payments, and advertising.

The latter sector is hotting up this week, with significant announcements from Google, AdMob and Greystripe.

Google first: it's expanding its AdSense for Mobile Applications beta, following tests with ten developers including Shazam and Urbanspoon.

It delivers ads for iPhone and Android apps, but to sign up for the public beta, apps must be free and generating at least 100,000 pageviews per day.

It's not clear how games fit into this (if, indeed, they do) - Google has separate services online for in-game ads, and that may turn out to be the case with iPhone too.

Meanwhile, AdMob has caused controversy with its decision to stop allowing its adverts to be delivered through multiple-network ad services like AdWhirl and Tapjoy.

"We have discovered that iPhone apps that use a mediation layer generate a substantially higher volume of complaints from end users and higher number of technical issues than apps that integrate with AdMob directly," blogs VP of product management Ali Diab.

AdWhirl and Tapjoy aren't happy, as you might expect. VentureBeat points out that the decision means AdMob will get to keep a bigger share of its advertising revenues.

Finally, Greystripe is fighting its corner with a new CPM Protection Program that claims to guarantee to beat any other iPhone ad network's eCPM by at least 25 per cent for 60 days.

The company is pitching its rich media Flash-like ads as a better option for iPhone games developers, because they attract higher rates.

There's a wider story going on here, of early movers like AdMob and Greystripe jockeying for position, while Google hovers ominously on the sidelines.

Developers certainly have no shortage of ad networks fighting for their advertising slots. Figuring out which will prevail in the long-term is a challenge, to say the least.