Sireet is a new Cydia tweak which allows jailbroken iPhone 4S users to send tweets through Siri. Sireet integrates itself to Siri, so after its installation you still have to open Siri to make it working, means no Sireet icon will display there on Springboard.
See also:
AssistantExtensions Sends Tweets, Search YouTube, Open Apps etc. through Siri
Jailbreak iPhone 4S with Absinthe
Jailbreak iPhone 4 S with Command Line Interface Corona
Users who want to try Sireet head to Cydia and visit modmyi repo.