You can specify a custom API root on a per-account basis. When adding an account, tap the gear icon to reveal the advanced configuration screen. This feature is useful to specify proxy servers in cases where Twitter.com is unreachable. You could leverage this to use alternative microblogging services with Tweetie, though this feature is not yet "officially" supported and may not work as expected.

You can specify your own custom image service in the Settings. See developer.atebits.com for more information.

You can specify your own URL shortening service in the Settings. See developer.atebits.com for more information.