I am not much of a Greasemonkey user, myself, but I understand the appeal. Clearly so does Google who, according to ReadWriteWeb, has added native support for the scripting system to its browser.
I had hoped Google might consider implementing support for Mozilla’s Jetpack system for doing much the same thing. However, as the article notes, Google has hired the creator of Greasemonkey, Aaron Boodman. It is hardly surprising, then, that they incorporated his technology. That and perhaps the fact that Greasemonkey is better established than Jetpack and has a larger community of contributors and scripts.
I haven’t hacked on either scripting extension, but part of me still wonders if there is enough common ground to perhaps extract out an overlapping, open specification that both systems could implement to make hackers lives easier.