I had an issue under 1P (3.8.15) under Safari 5.0.5 where I could not get either Keychain or 1P to auto-enter the password under HTTP Auth schemes. Since this is Safari 5.0.5, the 1P button did appear, but I had to manually select the appropriate account each time. What really killed me is that the HTTP Auth dialog did not have the option to "store this password in the keychain". The checkbox just wasn't there. Every time I relaunched the browser I'd have to re-enter it.
I finally tracked this down to extraneous code left over from my last version of 1P. I upgraded from version 3.6.something to 3.8.15 without any intermediate versions; as a result I had both the 1P "scripting" button and the 1P "extension" button. I'd had to disable the 1P extension so they didn't get into a fight about it.
Today I located the "scripting" button (in ~/Library/Application Support/1Password/Extensions) and blew away all of them. ("30868", "31057", and "31267".) Restarting Safari I have the "store this password in keychain" button again, and it actually works between sessions again.
Anyway, failure to remove the old scripting extension on upgrade seems to be a bug in 1P 3.8.15.