I am using this experiment since its public release, but today I got a sad yellow message "The experiment you're trying to access is no longer available.", although it is still displayed at the http://www.google.com/experimental/ .
Clearing cookies and re-enabling this does not help.

http://dl.dropbox.com/u/1571982/g/google-experimental-2010-05-05.png
http://dl.dropbox.com/u/1571982/g/google-keyboard-shotcuts-experiment-discontinued-2010-05-05.png

Is it time to return to the trusty "Google Ctrl-Arrow" userscript?
http://userscripts.org/scripts/show/6575 ?

I have the same problem...

<< What happened to that cool thing I was playing around with last week?

The prototypes on Google Labs are meant to be low-maintenance experiments. If one disappears it may be because no one was interested enough to use it, or because it wasn't stable enough for users to try it out, or because it was so wildly successful that heavy usage brought the server to its knees. Don't worry — while that particular application may not reappear, there will be something just as interesting to replace it shortly. >>

http://www.googlelabs.com/faq#oldprojects

mbegin: yes, that makes perfect sense, but it does not explain, why the "Check out Google's latest ideas" page still (2010-05-05T18:27:11) displays it as a first option, when it actually does nothing but the yellow message.
The second experiment at least "works" (I cannot determine, whether well or not).

Update (2010-05-06T12:15:50+02:00) it actually works again, yay!

It could have been a temporary removal, while the team had to reconfigure something more above the normal upgrade-component routines.

All the same, it raises an interersting question, how does anyone but certain-in-the-know Googlers determine the actual version of any major Gmail component, or of Gmail as such?

My guess is they had to change some things to make it work with the new interface: http://blogoscoped.com/forum/170545.html