I'm back from my appointment. It took a long time to check in and a long time to wait to be seen, and then about an hour for the procedure - but it's finally done. Phew!
The first part of the exam was a transvaginal ultrasound. The technician could readily see that I have a partially bicornuate uterus and commented about that. Otherwise, I looked normal in that part of the exam.
After he was done with that they began the procedure to fill up my uterus with saline. That took a while to set up, but not so long to actually do. Once they were in there they took a couple 3D ultrasound images as well as some 2D ones. The news was pretty much all good - no polyps or fibroids, definitely an arcuate uterus (the top or "fundus" of my uterus is concave instead of convex or flat), with a very small, teensy weensy septum. The doctor said she absolutely wouldn't worry about it because it was so small, and it may not even be considered a septum - there is a specific length that it has to be before it will be defined as such. An arcuate uterus is considered a variation of a normal uterus and shouldn't cause miscarriages, although some studies have found it more likely to cause them in the second trimester.
The whole thing wasn't too painful; there was definitely some strong cramping when they injected the saline (you could tell my uterus was like, WTH?). I was glad that Jonathan was there to drive me home, though, because I was shaking like a mouse when I was done (too nervous and cold and tired from holding my legs up).