While Google marketers were using the Mobile World Congress as a mobile coming-out party, Google developers were busy releasing a significant new version of the Android software developer kit.

The new SDK, delayed several weeks as Google took time to incorporate developer feedback, represents a significant advance for the mobile operating system. While most changes are under the covers, the release includes a major refresh of the Android user interface, which seems to be moving to a tile-based arrangement similar to the iPhone. Because Android is extremely customizable, however, developers aren’t locked into that user interface.

Other Android advances include new capabilities for local search and mapping, new media codes and an updated plug-in that lets developers code Android apps with the popular Eclipse development tool. Google also debuted a bug-tracking database, which will aid third-party developers building Android apps.

Still looming: the highly anticipated release of Apple’s iPhone SDK, which CEO Steve Jobs promised to deliver before the end of February. While a few Mac rumor sites had some premature information, the SDK was still MIA at press time. Although the latest official iPhone update enables users to link from the home screen directly to Web applications, a formal SDK will let developers build native iPhone apps.