Mobile advertising firm Millennial Media this morning released its ‘Mobile Mix’ report covering its findings for the first quarter of this year. The report looks at trends in ad impressions across mobile devices and operating systems.

The mobile advertising company found that tablets – or ‘non-phone mobile devices’ – accounted for 20 percent of all 45 billion monthly ad impressions on its platform during Q1 2012, up from 15 percent in the same period last year.

The report also found that Android remained the top overall OS on the platform in the first quarter of this year, with a 49 percent share of impressions.

Furthermore, Millennial says Android made up 14 of the ‘top 20′ mobile phones.

Apple remained king of the hill when looking at individual mobile device makers thanks to its iPhone, however, with Samsung in second place with 4 of the top 20 mobile phones. Surprise: the BlackBerry Curve is ranked in second place.

Millenial says Apple represented 28 percent of the top manufacturers’ impression share, with the iPhone accounting for 15 percent of mobile ad impressions.

The Apple iPad, Samsung Galaxy Tab and Amazon Kindle Fire were the top tablets in Q1 2012, and all three were among the top 20 mobile devices.

Also read: New iPad overtakes original iPad in the US after only 2 months: Report

A report from MediaPost today claims that Apple is planning its own mobile advertising platform which could be called iAd.

Details of how the system will work are scarce but the company will supposedly be unveiling it on 7 April. It’s likely that, if the system is real, it will have something to do with Quattro Wireless, the mobile ad firm Apple recently acquired.

If Apple is launching its own ad network it would be another element in the tightly-knit ecosystem that has built up around the iPhone, iPod Touch and (soon) iPad.

Apple already makes money from sales of devices and apps. If it can make money from advertising to you on your mobile devices too, it will have an even more neatly-packaged, lucrative ecosystem than it already does. It will have achieved something no other mobile handset manufacturer ever has – a complete system of virtually guaranteeing income from consumers long after they’ve bought a device.

Whether consumers will actually want to be advertised to is another matter. Apple’s well marketed, user-friendly mobile universe is hugely appealing to users. If advertising gets thrown into the mix in too significant a way it would be likely to taint public opinion of the whole ecosystem.

While Apple is generally brilliant at launching new hardware, it has tended to handle supporting services less well. MobileMe was a PR disaster at launch, while the problems with the approval processes in the iPhone App Store are stuff of legend.

Then there’s the fact that the move would put Apple in direct competition with Google’s main business. Relations between the two companies are fraught at best right now. Apple entering the advertising market could ruin their relationship forever.

If the iAd is a real prospect, Apple had better handle this one carefully.