Command-T has begun opening a new tab rather than a new window (actually, I think it's again), but it most definitely wasn't when I posted; I tried it multiple times and each time it opened a new window loaded with the website I had visited the previous time it opened a new window.
I might have quit and relaunched Safari in the meantime.
Command-shift-T does nothing.
Hmmm... I just noticed this: under "File", command-T is "New Tab", and under "History" it's "Reopen Last Closed Tab" (which command-Z used to do).
Dueling commands?
I give up.
