I'm on Straight Talk, and both my N5 and N7 (WiFi only) got the update around 3 yesterday (Thursday). I'm guessing that both units were randomly selected at the same time for the next batch of updates that were rolled out. We are probably at the the 50% of users stage of the roll out (Rolls out to 1%, then 25%, then 50%, then 100%). Doesn't seem like an update that needs to be pulled and fixed, so full release to all Nexus devices should be soon.
Like it was said above, with a Nexus device, you get the update from Google and it doesn't matter what carrier you are using. The only things that are Straight Talk centric on our phones is the SIM card and the radio settings to get on their network for cell and data access. Just access to their pipes (AT&T in my case) and nothing else.
For the record...I was at home on WiFi at the time in received the update availability notification, so the connection on my N5 to Google was via WiFi and not Straight Talk.