What's 'set_session_login' ?

I'm running the 8/22 Beta with minimal customizations at this point ...

I noticed that there are links generated via zen_back_link() that include "&set_session_login=true" in the URL, what's up with that?

I don't yet have a scenario pinned down to recreate this at will, but I've seen it a couple of times.

This variable is an artifact, left over from versions as early as 1.2.0. It's not connected to anything and is completely harmless. It will be removed in a future release, nothing to be concerned about.

As blindside says, it's been there since v1.2.0
But it probably will not be removed, as it does serve a purpose, albeit isolated: it only fires in the case of using customer-authorization restrictions.
It will likely be revamped in v2.0 with a more sophisticated and less GET-unfriendly approach.

DrByte:

it only fires in the case of using customer-authorization restrictions.

Ah, that's why I haven't seen it before: I'm putting together a new site that is "Customer Shop Status: Must login to browse (1)" and that's where I'm seeing it.