I'm at a conference using their (open) wifi. Normally I don't bother to tunnel my connection because I figure anything I care about (email, banking, etc.) goes over SSL/HTTPS anyway. It occurred to me, though, that my Plus client is set to automatically upload all my photos anytime it finds wifi. Are those photos going "in the clear"?
I have verified that Google+ version 18.104.22.1681037 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 22.214.171.124, but remember this may change) on HTTP port 443. Using Wireshark, it's clear that the traffic is encrypted.
It is hard to tell which protocol is being used in the G+ app (especially since Google is trying out new protocols like SPDY lately).
This is by no means a guaranteed approach, but you may be able to take some steps to answer this one yourself using information from the SO post Monitor network activity in Android Phones. They recommend using a tool called Shark for Root which seems to be like Wireshark for Android handsets. Please note, that if you aren't very network savvy, the output may be difficult to interpret and will probably require an install of Wireshark on a PC to interpret.
Note: If I get a chance in the next few days, I'll try out this approach myself and update the answer.