According to T-Mobile, that 5.1 OTA should start rolling out today:
Android Police has a nice run-down of expected new features (in addition to the usual bug fixes and stability improvements): http://www.androidpolice.com/tags/android-5-1-feature-spotlight/
Beginning March 9, the Google Nexus 4 received a software update to Android version 5.1 / LMY47D available via Over-the-Air (OTA) pull down over Wi-Fi only. This update will be available over cellular network at a later date and provides features and improvements
Android Police has a nice run-down of expected new features (in addition to the usual bug fixes and stability improvements): http://www.androidpolice.com/tags/android-5-1-feature-spotlight/