<--- I'd like to be able to see the date someone signed up, without having to visit their profile page.
you can, essentially.
just put your mouse over a user's handle, on the left side of their post - then, on the status bar at the bottom left of your browser window (assuming you have the Status Bar enabled in "View"), that users registration number will show up in the form: "u=xxxxx (some number)"
for example, your number is 7361. mine is 7795. indicating that i signed up 434 user registrations after you did.
sirius' (the OP) user number is 4 - indicating that he's been here for awhile...