Seems to be a bit of a chinese influence on some of our warm up techniques. One of my facourites is similar to something you probably do already - feet 2 shoulder widths apart, knees bent, facing forwards. Stretch in front of you, then describe a large circle with your arms, moving, with the bend at your waist (so half way through you are leaning backwards). Do 4 in each direction, slowly and stretching as far as possible.
I'd heard that Ueshiba didn't do many warm up excercises as he expected you to do them before class (maybe later on). A good warm up is ai-hanmi ikkyo, where uke pushes through and you move your body around this centre point to do a very blending style ikkyo. Not only does it get you into blending, but it gently warms up your back, wrists and toes. (won't work if force used!)
P.S. A recent scientific study has suggested that cardiovascular excercise is better for warm ups (to avoid injury) than stretching.