Admob hit (what I think is) a big milestone in May. They are now serving 100M ad impressions each day to cell phones. (See their May Metrics report) The US represents 45 percent of this traffic. Admob serves ads on to 5000 publisher sites. They are not the only mobile ad network in the US so the number of daily impressions is actually even higher. It's still a small fraction of the online impressions, but shows the potential of this market.

Their May Metrics report just came out and it's full of even more detail than ever before - more countries, top handsets by country, etc. - a wealth of information and must read.

We're seeing a confluence of factors contribute to this growth. Here's a few:

• Smartphone sales this year are far ahead of last year's despite a bit of an economic downturn
• Flat rate data plans / less expensive data plans
• More people on data plans
• Increased adoption of cell phones by teens
• Apple's iPhone - good handheld browsing experience (I'm expecting a large uptick in traffic when the 3G version goes on sale next month) - the iPhone already ranks 12th on the list of handsets in the US despite relatively small marketshare
• Popularity of social networking
• Better UI's
• 3G adoption / Wi-Fi in handsets

We'll be writing a lot more on this topic this year. Stay tuned.