One reason why Google and other companies develop interfaces optimized for iPhone is that people actually use Apple's phone to browse the web. New York Times (free login) reports that "on Christmas, traffic to Google from iPhones surged, surpassing incoming traffic from any other type of mobile device (...). A few days later, iPhone traffic to Google fell below that of devices powered by the Nokia-backed Symbian operating system but remained higher than traffic from any other type of cellphone."
Google has another reason for providing special interfaces for iPhone: they'll work on Android phones too, since Android's default browser uses the same rendering engine as Apple's Safari, namely WebKit.
Today, in the first day of MacWorld, Google will announce an update for its unified mobile interface, codenamed Grand Prix. Among the new features, the navigation bar will be customizable, Gmail will automatically show new messages without having to refresh the page, the compose page will include contacts, while Google Calendar will add a month-at-a-glance view. The updated iGoogle will probably be integrated in the interface.
"Google, which developed the first version of Grand Prix in six weeks, is introducing a new version on Monday, just six weeks after the first one. That is a speed of development not previously possible on mobile phones," said Vic Gundotra, vice-president at Google.
Vic Gundotra expects that "consumers are going to demand Internet browsers [as good as Apple's]" and the mobile web experience will improve.
Update.
From a Google announcement (my emphasis): "These new features provide iPhone users with a desktop-like Google web application experience in terms of ease-of-use, speed, and feature richness but optimized for the iPhone. This experience is made possible by the iPhone's general usability and the capabilities of its web browser, combined with Google's innovative mobile web applications. We plan to expand this experience to international versions of the iPhone and to other platforms that offer similar usability and browser capabilities. One of our goals is to support platforms that are fulfilling the promise of the mobile web - like the iPhone - and to ultimately deliver unique and compelling mobile experiences that improve people's daily lives."
Update 2: Google Mobile Blog has some screenshots of the updated interface.