We are planing to do a Tracker upgrade (with new features including API V3 and some integrations) tomorrow, Saturday Jan 23, at 5:30pm PST. We expect the upgrade to take under two hours.
Also, Engine Yard will be performing some maintenance and upgrades our hardware cluster on Sunday, from 10:00am to 1:00pm PST.
Sorry for the inconvenience. If you need access to your project data during the downtime, we recommend you export your project(s) beforehand:
http://www.pivotaltracker.com/help#howcaniexportstoriesfrommyproject
Also, as a reminder, the first version of the API (V1) will be disabled as part of tomorrow's release. More information on that here.