According to Sprint, the Nexus 5 and Nexus 6 should be receiving build LMY48I beginning today, with security fixes for the Stagefright exploit in tow. Being as there is no specific SKU for Sprint on either of these phones, the update should apply to all Nexus 5 and Nexus 6 models.
While carrier documents are not always the most accurate source of information, it isn't too far fetched that Google would be pushing an update this quickly. Google has yet to post the new images, so be sure to keep an eye on your phone for the update. If you have already received the update, be sure to let us know in the comments below.
Newer model: Nexus 6P