Recently PR #3122 was merged, which lays the foundation for adding AdSense to Condenser, which powers Steemit.com. It lays the foundation for displaying Google Ads on Steemit. Currently it just adds a GoogleAd element, and loads AdSense code on Condenser. However, soon (most likely) ads will soon be displayed in posts and other locations.
The PR also adds (no pun intended (fine, I did intend the pun)) the ability to have ads selectively shown or not shown, based on factors that have not yet been decided. Perhaps users over a certain reputation, or perhaps all logged in users will get an ad-free experience.
Personally, I don't find this very surprising, as @ned has said that Steemit is running out of funds, due to the low price of Steem. Adding ads to Steemit might be an effective way to fund Steemit in the future. Content creators might not like having ads shown with their content, however. Additionally, Steemit using this content for commercial purposes might pose legal issues (disclaimer: IANAL). The Steemit Terms of Service doesn't grant Steemit a licence to content on the blockchain, and not everyone on the blockchain has agreed to the Steemit ToS.
Keep in mind that this just impacts Steemit. Other dApps/interfaces, such as Busy won't be affected. This might result in Steemit.com competitors having another advantage over Steemit.com.
thnx for ads - now market,transfers not working! i cant even see my private key:D
lol... took me waaaay too long to figure out what you meant there :P