Where's Opera on SACD???

There's la Boheme on Decca with Angela Georghiu and her husband. The performance is truly very good. The mixing, only acceptable.
The "Poisoned Kiss" by Vaughan Williams just came out on Chandos. Quite enjoyable alla G&S way.
I just purchased a DVD-A version of Die Walkure on Farao Classics. They have some long excerpts on the website. This recording comes from a 2002 production at the Bavarian conducted by Zubin Mehta. I'm not a fan of ZM but this time he does well. Waltraud Meier, Peter Seiffert, John Tomlinson and Mihoko Fujimura (watch her!) sang their heart out. Gabriele Schnaut is OK. I got mine at Virgin Records.

From my perspective Opera is much better served on DVD than CD or SACD. It's an audio-VISUAL extraviganza. Hell, for me it was worth setting my system up as a home-theater just to experience opera. Now you can get two versions of the full Ring cycle on DVD, and each can be had for around $100. One's by The Met (they've got three cycles of that particular production coming up next year by the way), and the other is the centenial performance at Bayreuth. And both are in 5.1 format. Siegfried is on two disks on both productions.

Some of Renee Fleming's work is available on SACD, though I guess its not really opera its pretty close.

That is a good question, it would be nice to get a SACD Carmen on a single disc!

La Boheme is on SACD (likely a pcm recording, on London); there may be some others as well, but I haven't checked it yet. The major labels likely would never take advantage of the storage capacity of the medium, for the reason you mention.