Besides adding new themes to promote the service, Google uses some other tricks to increase iGoogle's usage. If you click on the iGoogle link from the homepage, Google sets a cookie preference so that every time you go to Google.com, you are redirected to iGoogle.
When you add a gadget from a web page to iGoogle, there's a vague setting enabled by default: "See this gadget when you visit Google.com", which also sets a cookie preference that redirects you to iGoogle.
The cookie preference is reverted when you click on "Classic Home" in iGoogle, but the effect of that extra click is not obvious.
"iGoogle is a more personal way to use Google.com. Customize your page anyway you like, by adding your favorite themes and gadgets from across the web." That's how Google describes the personalized homepage of today, the social network of tomorrow.
For those who use both the classic homepage and iGoogle, but would rather see the classic homepage when they go to Google.com, the only solution is to type in the address bar http://www.google.com/ig or http://www.igoogle.com instead of clicking the iGoogle link.
Google redirects people that visit Google.com to different pages, based on their locations or devices, but it would be nice to explicitly define the preferences. Windows doesn't change the default browser to Internet Explorer just because you accidentally clicked on the IE icon.