Does the official Google Plus Android client use HTTPS for automatic photo upload?
I have verified that Google+ version 2.4.1.271037 on Android 4.0.3 uses HTTPS for instant upload. 

I did what @shambleh suggested and performed a network trace at my Internet gateway. Enabled instant upload, snapped a picture, and saw a bunch of traffic hitting Google servers (IP address 74.125.226.70, but remember this may change) on HTTP port 443. Using Wireshark, it's clear that the traffic is encrypted.