Google officially announced the coming of Android 5.1 Lollipop earlier this afternoon, followed by a statement in the same blog post that they would be pushing it out to devices today. T-Mobile confirmed soon after that they would be pushing out Android 5.1 Lollipop to the Nexus 4, Nexus 5, and Nexus 7 2013 models on their network beginning today as well. While the updates may begin today(even though there is not much day time left)Google had yet to start pushing the Android 5.1 Lollipop source code up to AOSP, which is where developers can go to grab all the necessary files and use the code to work with and create awesome custom ROM experiences based off of the latest Android version of software, as well as use the code to make any changes or updates to their apps.
Google has just now started pushing the Android 5.1 Lollipop to the Android Open Source Project, so anyone interested that is capable of working with the source code will be able to get what they need very soon. Google will be continuing to push out the source code to AOSP as the day goes on, so even though it may not all be there as of right now chances are that by tomorrow morning everything may be in the AOSP repositories.
In addition to the source code going up to AOSP, Google is also pushing out the factory images for the Nexus devices it looks like, so Android 5.1 Lollipop is certainly in full swing as of this moment. The source code will essentially be unimportant to end users as they have no need for it and won't be able to do anything with it. However, this is how developers get apps and ROM's and other bits of custom software that are working with Android updated with the latest changes, and eventually when things get implemented by the developers, end users will end up seeing those changes reflected down the line so it does connect to them in a round about way. If you want to grab the source code for what's available so far, you can do so by heading to the repository link from the source below.