We'd love to. We have a mission to have the Wiring framework working on as many microcontrollers as possible. Arduino uses Atmel controllers, and we support about 80% of the ATmega controllers (in Wiring 1.0).
Arduino forked the Wiring source in 2005. Wiring started as Hernando Barragan's thesis project in 2003, and Massimo Banzi was Hernando's thesis supervisor, along with Casey Reas of Processing. In 2005, Massimo co-founded Arduino by forking the Wiring source and creating some affordable hardware.
Since then, the Arduino and Wiring API/Framework and IDE have not deviated very much at all, and we have asked the Arduino guys if they want to merge back so that we can focus on the software, and they can focus on their hardware (after all, they make the money on the hardware, not the software).
The response has been pretty quiet, and we often see the struggles that they have in maintaining their fork of the source. I think it would be fantastic if we could bring all of our strengths together, instead of working on two forks of the same software project.
It would be great if more people would ask the question to the Arduino guys, and not us! We are totally ready, willing, and able to work on the software with them/for them. It's just a matter of convincing them!