It's Greasemonkey, not GreaseMonkey.

Greasemonkey is a third-party extension. It wasn't created and isn't maintained by Mozilla, so it can't be officially obsoleted by Jetpack.

That said, if Jetpack supported transparently installing Greasemonkey scripts (the way Chrome does -- http://mashable.com/2010/02/01/google-chrome-greasemonkey-2/), it would effectively obsolete Greasemonkey because there'd be no need to install it.

Until then, there are 40k user scripts on userscripts.org that still need Greasemonkey installed in order to work: http://userscripts.org/.

Users decide whether something is obsolete, not creators. "Obsolete" refers to something that is antiquated, even if it is still in good working order. From Wikipedia: "Technical obsolescence may occur when a new product or technology supersedes the old, and it becomes preferred to utilize the new technology in place of the old."

As far as I know, Greasemonkey essentially runs user-selected javascript on a page automatically. The impression I get is that Jetpack does the same thing. Therefore, there will be no need for a third-party extension to achieve this functionality. Therefore, Greasemonkey will be "unnecessary," rendering it obsolete, regardless of any special edge cases. (Even 40K user scripts can be considered special edge cases - the set of actual implementation is much smaller than the set of possible javascript implementations.)

"Obsolete" may be an overloaded term, but this is the plain English usage, and it's the usage I'm using.

EDIT: I misunderstood that Jetpack was originally an extension and not just a codename for a new feature.

Fair enough. I misinterpreted your question to be "So, Mozilla says that this replaces Greasemonkey?", and wanted to clarify the situation (and also snark about the capitalization, a long-time peeve that I should really let go, but never quite can).