Apple today seeded the second beta of the upcoming OS X Yosemite 10.10.1 update to registered developers and participants in the AppleSeed program. The update is available for these users in the Software Update tab of the Mac App Store.
As with the first beta, Apple asks developers to focus their testing on WiFi-connectvitiy, Exchange accounts in the Mail app, and Notification Center. Apple has not indicated when OS X 10.10.1 will be released to the public, but the first point updates to OS X typically arrive within a month of the original version.