Stephen O' Grady has a blog post entitled Gmail Fighting Off Greasemonkey? where he writes

I'm not sure of the reasoning behind it, but it appears that Google may have made some behind the scenes changes to Gmail that disrupted the scipts and extensions I rely on to get the most out of my account. One of the Apache devs noted here that his Greasemonkey enabled persistent searches were no longer functioning, and in the same timeframe I've lost my delete key. It's not just Greasemonkey scripts, however, as my Gmail Notifier extension for Firefox has apparently been disabled as well. While it's Google's perogative to make changes to the service as necessary for maintenance or other reasons, I'm hoping this is not a deliberate play at preventing would-be participants from enhancing the value of Google's service. It's remarkable how much less useful Gmail is to me when I have to log in to see if I have mail, or can't easily delete the many frivolous emails I receive each day (yes, I'm aware that I can use a POP client for this, but I'd rather not).
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Update: As one reader and a few posters to the GM listserv have noted, one thing that's disrupted a variety of user scripts has been the fact that the domain to Gmail has changed from gmail.google.com to mail.google.com. While simply adding the domains into the GM interface had no effect on my Gmail, a reinstallation of a version of the script with updated domain returned my beloved Delete button. What do developers think of this change with Google's service? Here's one take from the GM list: "I noticed [the domain change] too. Why they can't just leave it alone, I can't understand." To be a bit less harsh, while Google probably had good reasons for making the change, it would have been great to see them be proactive and notify people of the change via their blog or some other mechanism.

I find this hilarious. Greasemonkey scripts work by effectively screen scrapping the website and inserting changes into the HTML. Stephen and others who are upset by Google's change are basically saying that Google should never change the HTML or URL structure of the website ever again because it breaks their scripts. Yeah, right.

Repeat after me, a web page is not an API or a platform.  

Speaking of apps breaking because of GMail domain changes it seems RSS Bandit users were also affected by the GMail domain name change. It looks like we have a problem with forwarding the username and password after being redirected from https://gmail.google.com/gmail/feed/atom to https://mail.google.com/gmail/feed/atom. I'll fix this bug in the next version but in the meantime RSS Bandit users should be able to work around this by changing the URL manually to the new one. My bad.