For a bit of a fun break from the work I've been doing recently for my research work, I've been hacking some new features for Blender. However, this evening, I ended up wasting quite a bit of time trying to figure out why the circle-select operator I'd been working on was not working. Namely, it wasn't responding to any events once activated.
Long story short: If you're coding a "Select Circle" operator, you need to add the following line at the end of gesture_circle_modal_keymap() in wm_operators.c:
This basically says that the modal keymap needed by these operator should be used when doling out events to your operator. Otherwise, the events simply won't get passed down, and you'll be left wondering why it's not working...