"In an effort to further increase security, in the coming weeks we'll be changing the main Google Calendar URL from 'www.google.com/calendar' to 'calendar.google.com.' Following the launch, the old Calendar url will simply redirect to the new one, so the overall impact of this change on Google Apps customers should be minimal," informed Google.
Browsers like Chrome and Safari will ask you for permission again to show notifications. Greasemonkey scripts and extensions that use the old URLs will probably stop working, so developers will need to update them.
Daniel Fletcher wonders why "Maps is now google.com/maps, when it used to be maps.google.com and Calendar seems to have gone the other way". Maybe Google Maps will go back to the old URL.