Nullriver has released a new version of Installer.app, 3.0b3, which makes significant enhancements to the software for downloading and installing third-party native applications directly on the iPhone (until February, anyway).
The new release is completely re-styled and now looks similar in some ways to the iTunes Wi-Fi Music application. Gone is the "one big list" interface -- items are now categorized in folders that can be browsed in the same method that the iPhone's iPod application uses to browse music. Categories include Games, Development, Health, Multimedia, Network, Productivity, and more. There is also now a "Recent Pacakges" category.
There are also some significant back-end enhancements, including the option to prompt for an iPhone SprinBoard restart if necessary, the ability to display only titles that are for the firwmare version your iPhone is running (1.0.2, 1.1.1, etc.), determine whether you are using an iPhone or iPod touch and show only appropriate appllications, and more.
Unfortunately, there is still no one-step jailbreak/push Installer.app function for iPhone firmware 1.1.1 via Nullriver's AppTapp. You can, however, use this process for Mac OS X, then simply update Installer.app from within the application itself.
iPhone firmware 1.1.1 is recommended for this release.