In Outlook Web Access' Settings, if you select "Always show From", a message with a grey background pops up that says...

Warning: Messages you compose won't be sent using S/MIME when this option is selected.

I have no idea why that matters. Does that mean you can see *and edit* the From line? That would be a little wacky, but even then, you just couldn't S/MIME if you edited it to an address that didn't match your key, right? That doesn't seem like a particularly difficult programming exercise.

I haven't been impressed with OWA yet. I have two accounts with it at work, and for the one that'll let me, I'm using ExQuilla and Thunderbird instead. OWA's web client is still entirely too rough around the edges.