There's this article (blog post?) in Time about how iPhone apps won't be free and it starts with:
"It's probably been a while since you bought new software. That's because so many tech firms — buoyed by ads placed in Web-based applications like the Google Docs word processor and the thousands of apps on Facebook — can now afford to give their programs away for free."
Are there really ads in Google Docs?
Contextual Advertising in PDF
Contextual Advertising in Word Documents
Advertising on CV